#include <bits/stdc++.h>
const uint64_t c_mod = 1000000007;
uint64_t ov[2048][2048];
uint64_t ps[2048];
int main() {
  std::ios_base::sync_with_stdio(false);
  int n, k;
  std::cin >> n >> k;
  std::string s;
  std::cin >> s;
  ov[0][0] = 1;
  ps[0] = 1;
  for (int i = 1; i <= n; i++) {
    for (int j = 0; j <= k; j++) {
      for (int p = 0; p < i && (p + 1) * (n - i + 1) <= j; p++) {
        ov[i][j] +=
            (('z' - s[i - 1]) * ov[i - p - 1][j - (p + 1) * (n - i + 1)]) %
            c_mod;
        ov[i][j] %= c_mod;
      }
      ov[i][j] += ((s[i - 1] - 'a') * ps[j]) % c_mod;
      ov[i][j] %= c_mod;
      ps[j] += ov[i][j];
      ps[j] %= c_mod;
    }
  }
  std::cout << ps[k] << std::endl;
}
