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

const long long MOD = 1000000007;

int main() {
	long long A; cin >> A;
	string B; cin >> B;
	long long N = B.size();

	vector<long long> A_pow(N, 1);
	for (long long i = 1; i < N; i++) {
		A_pow[i] = A_pow[i - 1] * A % MOD;
	}

	long long ans = 0;

	for (long long i = 1; i < N; i++) {
		ans += (i - 1) * (A_pow[i] - A_pow[i - 1]) % MOD;
	}

	long long b = 0;
	for (long long i = 0; i < N; i++) {
		b += A_pow[i] * B[N - i - 1] % MOD;
	}
	b %= MOD;

	ans += (N - 1) * (b - A_pow[N - 1] + 1) % MOD;

	cout << (ans % MOD + MOD) % MOD << endl;
}