#include <bits/stdc++.h>
long long cnt[2022][2022], total[2022], ans;
char s[2022];
int N, K, i, j, p, extra;
int main() {
  scanf("%d %d %s", &N, &K, s + 1);
  cnt[0][0] = total[0] = 1;
  ans = cnt[0][K];
  for (i = 1; i <= N; i++) {
    for (j = 0; j <= K; j++) {
      cnt[i][j] = (total[j] * (s[i] - 'a')) % 1000000007;
      for (extra = N - i + 1, p = i - 1; p >= 0 && extra <= j; p--) {
        cnt[i][j] = (cnt[i][j] + cnt[p][j - extra] * ('z' - s[i])) % 1000000007;
        extra += (N - i + 1);
      }
    }
    for (j = 0; j <= K; j++) total[j] = (total[j] + cnt[i][j]) % 1000000007;
    ans = (ans + cnt[i][K]) % 1000000007;
  }
  printf("%I64d\n", ans);
  return 0;
}
