class Solution {

    public int romanToInt(String S) {

        int ans = 0, num = 0;

        for (int i = S.length()-1; i >= 0; i--) {

            switch(S.charAt(i)) {

                case 'I': num = 1; break;

                case 'V': num = 5; break;

                case 'X': num = 10; break;

                case 'L': num = 50; break;

                case 'C': num = 100; break;

                case 'D': num = 500; break;

                case 'M': num = 1000; break;

            }

            if (4 * num < ans) ans -= num;

            else ans += num;

        }

        return ans;

    }

}