diff --git a/gamecore.cpp b/gamecore.cpp
index ed3f265..5dbd550 100644
--- a/gamecore.cpp
+++ b/gamecore.cpp
@@ -1,8 +1,10 @@
 #include "gamecore.h"
 #include "drawutils.h"
+#include "mathutils.h"
 #include "debug.h"
 
-int GameCore::NODE_SIZE = 25;
+int GameCore::MAX_MOVE_DISTANCE = 100;
+int GameCore::NODE_RADIUS = 12;
 
 GameCore::GameCore()
 {
@@ -73,14 +75,14 @@ void GameCore::render()
     {
 	Vertex* v = graph.get_current_vertex();
 	DrawUtils::draw_circle_filled(display, v->x, v->y,
-				      Graph::MAX_MOVE_DISTANCE, 0xcb1919, 128);
+				      MAX_MOVE_DISTANCE, 0xcb1919, 128);
     }
 
     for (list<Vertex*>::iterator cursor = vertices.begin();
 	 cursor != vertices.end(); cursor++)
     {
 	Vertex v = *(*cursor);
-	DrawUtils::draw_circle_filled(display, v.x, v.y, NODE_SIZE>>1,
+	DrawUtils::draw_circle_filled(display, v.x, v.y, NODE_RADIUS,
 				      0x000000);
     }
 
@@ -118,5 +120,11 @@ void GameCore::on_exit()
 
 void GameCore::on_lbutton_down(int x, int y)
 {
-    graph.do_vertex(x, y, NODE_SIZE);
+    Vertex* cv = graph.get_current_vertex();
+    if (cv != NULL &&
+	(MathUtils::distance(cv->x, cv->y, x, y)
+	 > MAX_MOVE_DISTANCE))
+	return;
+
+    graph.do_vertex(x, y, NODE_RADIUS);
 }
diff --git a/gamecore.h b/gamecore.h
index 3a79b72..1f4181f 100644
--- a/gamecore.h
+++ b/gamecore.h
@@ -41,8 +41,8 @@ class GameCore : public MainEvent
     // data
     Graph graph;
 
-    // constants
-    static int NODE_SIZE;
+    static int NODE_RADIUS;
+    static int MAX_MOVE_DISTANCE;
 };
 
 #endif
diff --git a/graph.cpp b/graph.cpp
index f079a53..1e8cae5 100644
--- a/graph.cpp
+++ b/graph.cpp
@@ -1,9 +1,6 @@
 #include "graph.h"
-#include "debug.h"
 #include "mathutils.h"
 
-int Graph::MAX_MOVE_DISTANCE = 100;
-
 Graph::Graph()
 {
     current_vertex = NULL;
@@ -19,41 +16,23 @@ Graph::~Graph()
     }
 }
 
-bool Graph::vertex_present(int x, int y, int size)
+bool Graph::vertex_present(int x, int y, int r)
 {
-    int delta = size / 2;
-
-    int x_min = x - delta;
-    int x_max = x + delta;
-    int y_min = y - delta;
-    int y_max = y + delta;
-
     for (list<Vertex*>::iterator cursor = vertices.begin();
 	 cursor != vertices.end(); cursor++)
     {
-	Vertex v = *(*cursor);
-	if (((x_min >= v.x_min && x_min <= v.x_max) &&
-	     ((y_min >= v.y_min && y_min <= v.y_max) ||
-	      (y_max >= v.y_min && y_max <= v.y_max))) ||
-	    ((x_max >= v.x_min && x_max <= v.x_max) &&
-	     ((y_min >= v.y_min && y_min <= v.y_max) ||
-	      (y_max >= v.y_min && y_max <= v.y_max))))
-	{
-#ifdef DEBUG
-    cerr << "debug: Graph::vertex_present(): vertex present at x=" << v.x << ", y=" << v.y << "\n";
-#endif
-	    return true;
-	}
+	Vertex* v = *cursor;
+	if (MathUtils::distance(v->x, v->y, x, y) <= v->r) return true;
     }
 
 	return false;
 }
 
 
-void Graph::do_vertex(int x, int y, int size)
+void Graph::do_vertex(int x, int y, int r)
 {
-    if (vertex_present(x, y, size)) select_vertex(x, y);
-    else add_vertex(x, y, size);
+    if (vertex_present(x, y, r)) select_vertex(x, y);
+    else add_vertex(x, y, r);
 }
 
 
@@ -63,7 +42,7 @@ void Graph::select_vertex(int x, int y)
 	 cursor != vertices.end(); cursor++)
     {
 	Vertex* v = *cursor;
-	if (x >= v->x_min && x <= v->x_max && y >= v->y_min && y <= v->y_max)
+	if (MathUtils::distance(v->x, v->y, x, y) <= v->r)
 	{
 	    current_vertex = v;
 	    return;
@@ -72,23 +51,12 @@ void Graph::select_vertex(int x, int y)
 }
 
 
-void Graph::add_vertex(int x, int y, int size)
+void Graph::add_vertex(int x, int y, int r)
 {
-    if (current_vertex != NULL &&
-	(MathUtils::distance(static_cast<float>(current_vertex->x),
-			     static_cast<float>(current_vertex->y),
-			     static_cast<float>(x),
-			     static_cast<float>(y))
-	 > MAX_MOVE_DISTANCE))
-	return;
+    Vertex* v = new Vertex(x, y, r);
 
-    Vertex* v = new Vertex(x, y);
-    v->x_min = x - size/2;
-    v->x_max = x + size/2;
-    v->y_min = y - size/2;
-    v->y_max = y + size/2;
-
-    if (vertex_present(v->x, v->y, 25)) return;
+    // Make sure the nodes won't overlap
+    if (vertex_present(v->x, v->y, v->r + r)) return;
 
     vertices.push_back(v);
 
diff --git a/graph.h b/graph.h
index 0aee507..a2ecd5e 100644
--- a/graph.h
+++ b/graph.h
@@ -14,15 +14,12 @@ using std::list;
 class Vertex
 {
   public:
-    Vertex(int x, int y)
+    Vertex(int x, int y, int r)
     { this->x = x; this->y = y; }
 
     int x;
     int y;
-    int x_min;
-    int y_min;
-    int x_max;
-    int y_max;
+    int r;
 };
 
 struct Edge
@@ -42,14 +39,12 @@ class Graph
     list<Vertex*> get_vertices() { return vertices; }
     list<Edge> get_edges() { return edges; }
 
-    void do_vertex(int x, int y, int size);
+    void select_vertex(int x, int y);
+    void do_vertex(int x, int y, int r);
     Vertex* get_current_vertex() { return current_vertex; }
 
-    static int MAX_MOVE_DISTANCE;
-
  private:
-    void add_vertex(int x, int y, int size);
-    void select_vertex(int x, int y);
+    void add_vertex(int x, int y, int r);
 
     Vertex* current_vertex;
     list<Vertex*> vertices;