#include <bits/stdc++.h>
int f[2005][2005], sum[2005][2005], n, m;
char s[2005];
int main() {
  scanf("%d%d", &n, &m);
  scanf("%s", s + 1);
  sum[0][0] = f[0][0] = 1;
  for (int j = 0; j <= m; j++) {
    for (int i = 1; i <= n; i++) {
      int val = n - i + 1;
      for (int k = i - 1; k >= 0; k--)
        if (j - val >= 0) {
          f[i][j] = (f[i][j] + 1LL * f[k][j - val] * ('z' - s[i])) % 1000000007;
          val += n - i + 1;
        } else
          break;
      f[i][j] = (f[i][j] + 1LL * sum[i - 1][j] * (s[i] - 'a')) % 1000000007;
      sum[i][j] = (sum[i - 1][j] + f[i][j]) % 1000000007;
    }
  }
  printf("%d\n", (f[n][m] + sum[n - 1][m]) % 1000000007);
}
