def roman(r):
    r_nums = {'M': 1000, 'D': 500, 'C': 100, 'L': 50, 'X': 10, 'V': 5, 'I': 1}
    num = 0
    prev_num = -1
    for n in r[::-1]:
        if prev_num <= r_nums[n]:
            num += r_nums[n]
        else:
            num -= r_nums[n]
        prev_num = r_nums[n]
    return num


print(roman('MMI'))         # => 2001
print(roman('XXI'))         # => 10
print(roman('MCMXVI'))      # => 1916
print(roman('XC'))          # => 90
print((roman('MCMLXV')))    # => 1965