#include <bits/stdc++.h>
using namespace std;
const int P = 1000000007;
int n, k, i, f[2005][2005], g[2005][2005], ch[2005], j, p, last;
char c;
int main() {
  scanf("%d%d", &n, &k);
  c = getchar();
  for (i = 1; i <= n; i++) scanf("%c", &c), ch[i] = c - 96;
  f[0][0] = 1;
  for (i = 1; i <= n; i++)
    for (j = 0; j <= k; j++) {
      g[i][j] = (f[i - 1][j] + g[i - 1][j]) % P;
      f[i][j] = g[i][j] * 1ll * (ch[i] - 1) % P;
      last = 0;
      for (p = i - 1; p >= 0; p--) {
        last += (n - i + 1);
        if (last > j) break;
        f[i][j] = (f[i][j] + f[p][j - last] * 1ll * (26 - ch[i]) % P) % P;
      }
    }
  printf("%d", (f[n][k] + g[n][k]) % P);
  return 0;
}
