#include <bits/stdc++.h>
int N, K, f[2005][2005], g[2005][2005];
char S[2005];
void doit() {
  scanf("%d%d%s", &N, &K, S + 1), f[0][0] = g[0][0] = 1;
  for (int i = 1; i <= N; i++)
    for (int j = 0; j <= K; j++) {
      f[i][j] = 1ll * g[i - 1][j] * (S[i] - 97) % 1000000007;
      for (int k = 0, t; k < i && (t = (k + 1) * (N - i + 1)) <= j; k++)
        (f[i][j] += 1ll * f[i - k - 1][j - t] * (122 - S[i]) % 1000000007) %=
            1000000007;
      g[i][j] = (g[i - 1][j] + f[i][j]) % 1000000007;
    }
  printf("%d\n", g[N][K]);
}
int main() {
  doit();
  return 0;
}
