Add two numbers present on sheet on User Alert using Google Apps Script

function addtwonumbersonsheet() {
 
var sh1=SpreadsheetApp.getActiveSpreadsheet();
var sheet1=sh1.getSheetByName("GAS_Sheet");
var num1=sheet1.getRange(2,1).getValue();
var num2=sheet1.getRange(2,2).getValue();  
var sh=SpreadsheetApp.getUi();
var response=sh.alert("Do you want to add the two number?",sh.ButtonSet.YES_NO);
if (response == sh.Button.YES)
{
  sheet1.getRange(2,3).setValue(num1+num2);
 
}

}