class Solution {
public:
    int minimumRecolors(string blocks, int k) {
        int ret = 0, len = blocks.size(), t = 0;
        for (int i = 0; i < k; i++)
            if (blocks[i] == 'W')
                t += 1;
        ret = t;
        for (int i = k; i < len; i++) {
            if (blocks[i] == 'W') t += 1;
            if (blocks[i-k] == 'W') t -= 1;
            if (ret > t) ret = t;
        }
        return ret;
    }
};