HW2E

#include <iostream>
#include <vector>

using namespace std;


int main () {
    const int MAX = 20000000;
    vector <bool> S (MAX, true);
    for (int k = 2; k * k <= MAX; k++) {
        if (S[k]) {
            for (int l = k * k; l < MAX; l += k) S[l] = false;
        }
    }

    int M, N;
    cin >> M >> N;
    int count = 0;

    for (int i = 1; i <= N; ++i) {
        count += S[i];
    }

    if (count == M) {
        cout << "1";
        return 0;
    }

    for (int i = N + 1; i < MAX; ++i) {
        count += (S[i] - S[i - N]);
        if (count == M) {
            cout << i - N + 1;
            return 0;
        }
    }
    cout << "-1";
    return 0;
}