IOI '98 P3 - Party Lamps

#include <iostream>
#include <string>

using namespace std;

int main() {
    int n, c, x;
    cin >> n >> c;
    string should(n, ' ');
    while (cin >> x && x != -1) should[x-1] = '1';
    while (cin >> x && x != -1) should[x-1] = '0';

    bool got = false;
    for (int bitmask = 0; bitmask < 16; ++bitmask) {
        int pressed = __builtin_popcount(bitmask);
        if (c < pressed || (c - pressed) % 2) continue;

        string s;
        bool works = true;
        for (int i = 0; i < n; ++i) {
            s.push_back('1');
            if (bitmask&1) s.back() = '0'+'1'-s.back();
            if ((bitmask&2) && !(i&1)) s.back() = '0'+'1'-s.back();
            if ((bitmask&4) && (i&1)) s.back() = '0'+'1'-s.back();
            if ((bitmask&8) && !(i%3)) s.back() = '0'+'1'-s.back();
            if (should[i] != ' ' && s.back() != should[i]) works = false;
        }
        if (works) got = true, cout << s << '\n';
    }
    if (!got) cout << "IMPOSSIBLE\n";
    return 0;
}