diff --git a/src/interpreter.c b/src/interpreter.c
index c6cff36..a85b0ee 100644
--- a/src/interpreter.c
+++ b/src/interpreter.c
@@ -1432,9 +1432,9 @@ GroundValue interpretGroundInstruction(GroundInstruction inst, GroundScope* scop
                     }
 
                     if (right->type == INT) {
-                        result += right->data.intVal;
+                        result -= right->data.intVal;
                     } else if (right->type == DOUBLE) {
-                        result += right->data.doubleVal;
+                        result -= right->data.doubleVal;
                     }
 
                     addVariable(scope->variables, in->args.args[2].value.refName, createDoubleGroundValue(result));