[WIP] [TOI 2020 4th] B. 期望長度

/* https://drive.google.com/open?id=1IC38qlefiVIW7Rm1pSWEdAiIMR5f-V1z */
/* O(n * 2^n) TLE, 32 pts */

#pragma GCC target("avx2")
#pragma GCC optimize("O3", "unroll-loops")

// #include <bits/extc++.h>
// using namespace __gnu_pbds;

#include <bits/stdc++.h>
using namespace std;

//#define int long long
#define double long double
// template <typename T>
// using pbds_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
using pii = pair<int, int>;
template<typename T>
using prior = priority_queue<T, vector<T>, greater<T>>;
template<typename T>
using Prior = priority_queue<T>;

#define X first
#define Y second
#define ALL(x) (x).begin(), (x).end()
#define eb emplace_back
#define pb push_back

#define fastIO() ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0)
#define RANDOM() random_device __rd; \
                 mt19937 __gen = mt19937(__rd()); \
                 uniform_int_distribution<int> __dis(1, 1E8); \
                 auto rnd = bind(__dis, __gen)

const int INF = 1E18;
const int mod = 1E9 + 7;

int32_t main() {
    fastIO();

    int n, N, num = 0, ans = 0;
    string s;
    cin >> n >> s, N = 1 << n;

    for (auto c : s) num = 2 * num + (c-'0');

    for (int i = 0; i < N; ++i) {
        int row = 0, x;
        for (int j = 0; j < n; ++j) {
            if (num >> j & 1) x = row |  i;
            else              x = row | ~i;
            row = x & ((x - ((row << 1) + 1)) ^ x);
        }
        ans += __builtin_popcount(row & N-1);
    }

    int gcd = __gcd(N, ans);
    N /= gcd, ans /= gcd;

    if (N == 1) cout << ans << "\n";
    else cout << ans << "/" << N << "\n";

    return 0;
}