羅馬數字 -! 整數

def roman_to_int(s):
    roman = {
        'I': 1, 'V': 5, 'X': 10,
        'L': 50, 'C': 100,
        'D': 500, 'M': 1000
    }
    total = 0
    prev = 0
    for ch in reversed(s):
        val = roman[ch]
        if val < prev:
            total -= val
        else:
            total += val
            prev = val
    return total

# 範例：
print(roman_to_int("MCMXCIV"))  # 1994