export function calculate(a: number, b: number, op: string): number | null {
  let res = null
  switch(op){
    case '+': res = a + b
    break
    case '-':res = a - b
    break
    case '*':res = a * b
    break
    case '/': res =a / b
    break
    default: res 
  }

  return res
}