diff --git a/main.cpp b/main.cpp
index 28d5b3a..712b5f0 100644
--- a/main.cpp
+++ b/main.cpp
@@ -121,13 +121,18 @@ Moves serialize(const string &s) {
         throw invalid_argument(err);
     }
 
-    result.x <<= 1;
-    result.y <<= 1;
+    result.x = (result.x << 1) - 1;
+    result.y = (result.y << 1) - 1;
 
     string player;
     int x, y;
     while (iss >> player >> x >> y) {
         if (player == "END") break;
+        if (x >= result.x || y >= result.y) {
+            string err = "Invalid move by: " + player + " (" + toString(x) + ", " + toString(y) + ")";
+            throw invalid_argument(err);
+        }
+
         if (!isValidPlayer(player[0])) {
             string err = "Invalid player: " + player;
             throw invalid_argument(err);