#include <bits/stdc++.h>
using namespace std;
const int MOD = 1e9 + 7;
const int N = 2222;
int n, K;
long long sum[N], tail[N], f[N][N];
char s[N];
long long work() {
  long long ret = 0;
  tail[n + 1] = 1;
  for (int i = n; i >= 1; i--) tail[i] = tail[i + 1] * (s[i] - 'a' + 1) % MOD;
  if (K == 0) return tail[1];
  f[0][0] = 1;
  sum[0] = 1;
  for (int i = 1; i <= n; i++) {
    int dt = n - i + 1;
    for (int j = 0; j <= K; j++) {
      for (int k = i - 1; k >= 0 && (i - k) * dt <= j; k--) {
        f[i][j] = (f[i][j] + f[k][j - (i - k) * dt]) % MOD;
      }
      f[i][j] = f[i][j] * ('z' - s[i]) % MOD;
    }
    for (int j = 0; j <= K; j++) {
      f[i][j] = (f[i][j] + sum[j] * (s[i] - 'a') % MOD) % MOD;
      sum[j] = (sum[j] + f[i][j]) % MOD;
    }
  }
  ret = sum[K];
  return ret;
}
int main() {
  scanf("%d%d%s", &n, &K, s + 1);
  printf("%I64d\n", work());
  return 0;
}
