POJ 1753 Flip Game

#include <cstdio>
#include <algorithm>
using namespace std;

const int INF = 0x7fffffff;
char s[20];
int change[16] = {19,39,78,140,305,626,1252,2248,4880,8992,20032,35968,12544,29184,58368,51200};
int pos[16] = {1,2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384,32768};

void solve()
{
    int init_state = 0;
    for(int i = 0; i < 16; ++i){
        if(s[i] == ‘b‘)
            init_state += pos[i];
    }
    int res = INF;
    for(int i = 0; i < 65536; ++i){
        int cnt = 0;
        int state = init_state;
        for(int j = 0; j < 16; ++j){
            if(i&pos[j]){
                ++cnt;
                state ^= change[j];
            }
        }
        if(state == 0 || state == 65535)
            res = min(res, cnt);
    }
    if(res == INF)
        printf("Impossible\n");
    else
        printf("%d\n", res);
}

int main()
{
    for(int i = 0; i < 4; ++i)
        scanf("%s", &s[i*4]);
    solve();
    return 0;
}