Transform a Roman format number in a decimal one (Python)

roman.py

n = 'MCMXCVII'  # 1997

def roman_to_decimal(n):
    roman_map = {'M': 1000, 'D': 500, 'C': 100, 'L': 50, 'X': 10, 'V': 5, 'I': 1}
    result = 0

    for i,c in enumerate(n):
        num = roman_map[c]
        
        if result == 0:
            result += num
        else:
            if num <= roman_map[n[i-1]]:
                result += num
            else:
                result -= roman_map[n[i-1]]
                result += num - roman_map[n[i-1]]

    return result

print roman_to_decimal(n)