#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <string>
#include <climits>
#include <functional>
#include "math.h"
using namespace std;
typedef long long ll;

int main() {
	int N, K;
	string A;
	cin >> N >> K;
	cin >> A;
	if (N == 1) {
		cout << 1 << endl;
	} else {
		vector<vector<ll> > cum(N + 1, vector<ll>());
		cum[1].push_back(0); cum[1].push_back(1); cum[1].push_back(1);
		int j_max = 2;
		for (int i = 2; i <= N; ++i) {
			ll sum_val = 0;
			cum[i].push_back(0);
			for (int j = 0; j < min(j_max, K + 1); ++j) {
				sum_val += (cum[i - 1][j + 1] - cum[i - 1][max(0, j - i + 1)]) % 1000000007;
				cum[i].push_back(sum_val % 1000000007);
			}
			j_max += i;
			for (int j = 0; j < i; ++j) {
				cum[i].push_back(sum_val % 1000000007);
			}
		}
		cout << cum[N][K + 1] % 1000000007 << endl;
	}
}