class Solution {
    public int minFlips(String S) {
        char[] s = S.toCharArray();
        char[] target = new char[]{'0', '1'};
        int n = s.length;

        int cnt = 0;
        for(int i = 0; i < n; i ++ ){
            if(s[i] != target[i % 2]){
                cnt ++ ;    
            }
        }

        int ans = Math.min(cnt, n - cnt);
        for(int i = 0; i < n; i ++ ){
            if(s[i] != target[i % 2]) cnt -- ;
            if(s[i] != target[(i + n) % 2]) cnt ++ ;
            ans = Math.min(Math.min(cnt,n - cnt), ans);
        }

        return ans;
    }
}