Error fix (#1)

javafx-template/src/main/java/app/Calc.java

@@ -38,10 +38,11 @@ public class Calc {
      * @throws IllegalArgumentException if n is larger than the operand count
      */
     public double peekOperand(int n) {
-        if (n > getOperandCount()) {
+        int index = getOperandCount() - n - 1;
+        if (index >= getOperandCount() || index < 0) {
             throw new IllegalArgumentException("Cannot peek at position " + n + " when the operand count is " + getOperandCount());
         }
-        return operandStack.get(getOperandCount() - n - 1);
+        return operandStack.get(index);
     }
 
     /**