input: Write a function to convert a number to its Roman numeral representation.
  assert Roman(1) == "I"
  assert Roman(4) == "IV"
  assert Roman(9) == "IX"
  assert Roman(58) == "LVIII"
  assert Roman(1994) == "MCMXCIV"
output: def Roman(num):
      val = [
          1000, 900, 500, 400,
          100, 90, 50, 40,
          10, 9, 5, 4,
          1
          ]
      syb = [
          "M", "CM", "D", "CD",
          "C", "XC", "L", "XL",
          "X", "IX", "V", "IV",
          "I"
          ]
      roman_num = ''
      i = 0
      while  num > 0:
          for _ in range(num // val[i]):
              roman_num += syb[i]
              num -= val[i]
          i += 1
      return roman_num