#include <bits/stdc++.h>
using namespace std;
const int N = 4e3 + 10, mod = 1e9 + 7;
char a[N];
int n, k;
long long dp[N][N], dp2[N][N];
int main() {
  cin >> n >> k;
  cin >> (a + 1);
  dp[0][0] = dp2[0][0] = 1;
  for (int i = 1; i <= n; i++)
    for (int j = 0; j <= k; j++) {
      dp[i][j] = dp[i - 1][j] * (a[i] - 'a' + 1) % mod;
      for (int t = 0; t < j / (n - i + 1) && i - t > 0; t++)
        dp[i][j] = (dp[i][j] +
                    ('z' - a[i]) * dp2[i - t - 1][j - (t + 1) * (n - i + 1)]) %
                   mod;
      dp2[i][j] = (dp[i][j] - dp[i - 1][j] + mod) % mod;
    }
  cout << dp[n][k];
  return 0;
}
