#include <bits/stdc++.h>
using namespace std;
int n, k;
char s[2005];
int f[2005][2005], op, ans, g[2005][2005];
int main() {
  scanf("%d %d", &n, &k);
  scanf("%s", s + 1);
  f[0][0] = 1, g[0][0] = 1;
  for (int i = 1; i <= n; i++) {
    for (int j = 0; j <= k; j++) {
      int o = (k - j) / (n - i + 1);
      for (int l = 0; l < i; l++) {
        if (l + 1 > o) break;
        op = 'z' - s[i];
        (f[i][j + (l + 1) * (n - i + 1)] +=
         1ll * f[i - l - 1][j] * op % 1000000007) %= 1000000007;
      }
      op = s[i] - 'a';
      (f[i][j] += 1ll * op * g[i - 1][j] % 1000000007) %= 1000000007;
    }
    for (int j = 0; j <= k; j++) g[i][j] = (g[i - 1][j] + f[i][j]) % 1000000007;
  }
  for (int i = 0; i <= n; i++) (ans += f[i][k]) %= 1000000007;
  printf("%d\n", ans);
  return 0;
}
