蓝桥--杨辉三角形

#include<algorithm>
#include<iostream>
typedef long long ll;
using namespace std;
int N;
ll combine(int n, int m) {
    ll ans = 1;
    for (int i = 0; i < m; i++) {
        ans *= (n - i);
        ans /= (i + 1);
        if (ans > N) {
            return ans;
        }
    }
    return ans;
}
bool f(int k) {
    int l = 2 * k, r = max(N, l);
    while (l < r) {
        int mid = (l + r) >> 1;
        if (combine(mid, k) >= N) {
            r = mid;
        } else {
            l = mid + 1;
        }
    }
    if (combine(r, k) == N) {
        cout << 1ll * (r + 1) * r / 2 + k + 1 << endl;
        return true;
    } else {
        return false;
    }
}
int main() {
    cin >> N;
    if (N == 1) {
        cout << "1" << endl;
    } else {
        for (int i = 17; i > 0; i--) {
            if (f(i)) {
                break;
            }
        }
    }
    return 0;
}