input: Write a python function to calculate compound interest.
  assert compound_interest(1000, 5, 2) == 1102.5
  assert compound_interest(1500, 4.3, 6) == 1938.8368221341054
  assert compound_interest(2000, 3.5, 5) == 2374.00934351222
output: def compound_interest(principal, rate, time):
      return principal * (1 + rate / 100) ** time