Roman number to decimal

def value(r):
    if (r == 'I'):
        return 1
    if (r == 'V'):
        return 5
    if (r == 'X'):
        return 10
    if (r == 'L'):
        return 50
    if (r == 'C'):
        return 100
    if (r == 'D'):
        return 500
    if (r == 'M'):
        return 1000
    return -1
 
def romanToDecimal(str):
    res = 0
    i = 0
 
    while (i < len(str)):
        s1 = value(str[i])
        if (i + 1 < len(str)):
            s2 = value(str[i + 1])
            if (s1 >= s2):
                 res = res + s1
                 i = i + 1
            else:
                res = res + s2 - s1
                i = i + 2
        else:
            res = res + s1
            i = i + 1
 
    return res

romannumber = input("Enter the Roman Number you want to convert: ")
print("Integer form of Roman Numeral is: " + str(romanToDecimal(romannumber)))