input: Write a function to repeat a string based on the integer value of another string.
  assert repeat('3', 'hello') == 'hellohellohello'
  assert repeat('2', 'world') == 'worldworld'
  assert repeat('5', 'test') == 'testtesttesttesttest'
output: def repeat(s1, s2):
      return s2 * int(s1)