diff --git a/AndroidCalculator/Calculator.swift b/AndroidCalculator/Calculator.swift
index 0da621c..dd96340 100644
--- a/AndroidCalculator/Calculator.swift
+++ b/AndroidCalculator/Calculator.swift
@@ -56,6 +56,12 @@ class Calculator {
         formula.append(formula.popLast()! + text)
       }
       
+    case "+", "-", "×", "÷":
+      if ["+", "-", "×", "÷"].firstIndex(of: formula.last!) != nil {
+        formula.removeLast()
+      }
+      formula.append(text)
+      
     default:
       print(formula)
     }