correction issue

src/main/java/fr/nancy/iut/CalcEngine.java

@@ -72,7 +72,7 @@ public class CalcEngine
         if (previousOperator == '+') {
             displayValue = leftOperand + displayValue;
         }
-        else {
+        else if (previousOperator == '-') {
             displayValue = leftOperand - displayValue;
         }
         leftOperand = 0;