#include <bits/stdc++.h>
const int N = 2001, INF = 1e9 + 7;
long long g[N][N], f[N][N];
int main() {
  int n, k;
  std::cin >> n >> k;
  std::string s;
  std::cin >> s;
  f[0][0] = g[0][0] = 1;
  for (int i = 1; i <= n; ++i)
    for (int j = 0; j <= k; ++j) {
      g[i][j] = f[i - 1][j] * (s[n - i] - 'a') + g[i - 1][j];
      f[i][j] = g[i][j];
      for (int p = 1; p <= i + 1 - p && p * (i + 1 - p) <= j; ++p) {
        f[i][j] += f[i - p][j - p * (i + 1 - p)] * ('z' - s[n - i + p - 1]);
        if (p < i + 1 - p)
          f[i][j] += f[p - 1][j - p * (i + 1 - p)] * ('z' - s[n - p]);
      }
      g[i][j] %= INF;
      f[i][j] %= INF;
    }
  std::cout << f[n][k];
  return 0;
}
