#include <bits/stdc++.h>
long long const mo = 1000000007;
int const maxn = 2007;
long long f[maxn][maxn];
long long sum[maxn][maxn];
int n, k;
std::string s;
int main() {
  std::ios::sync_with_stdio(false);
  std::cin >> n >> k;
  std::cin >> s;
  f[0][0] = sum[0][0] = sum[1][0] = 1;
  for (int i = 1; i <= n; i++) {
    for (int j = 0; j <= k; j++) {
      f[i][j] = ((s[i - 1] - 'a') * sum[i - 1][j]) % mo;
      for (int p = 0; p < i; p++) {
        if (j < (n - i + 1) * (p + 1)) break;
        f[i][j] += f[i - p - 1][j - (n - i + 1) * (p + 1)] * ('z' - s[i - 1]);
        f[i][j] %= mo;
      }
      sum[i][j] = (sum[i - 1][j] + f[i][j]) % mo;
    }
  }
  long long ans = 0;
  for (int i = 0; i <= n; i++) ans = (ans + f[i][k]) % mo;
  std::cout << ans % mo << '\n';
}
