#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:36777216")
using namespace std;
int n, m;
char ch[2222];
long long f[2222][2222], sum[2222][2222];
int main() {
  cin >> n >> m;
  scanf(" %s", ch + 1);
  f[0][0] = 1;
  for (int i = 0; i <= n; i++) sum[i][0] = 1;
  for (int i = 1; i <= n; i++) {
    for (int j = 0; j <= m; j++) {
      f[i][j] += sum[i - 1][j] * (ch[i] - 'a');
      f[i][j] %= 1000000007;
      for (int k = i - 1; k >= 0; k--) {
        if (j - (i - k) * (n - i + 1) < 0) break;
        f[i][j] += f[k][j - (i - k) * (n - i + 1)] * ('z' - ch[i]);
        f[i][j] %= 1000000007;
      }
      sum[i][j] = sum[i - 1][j] + f[i][j];
      sum[i][j] %= 1000000007;
    }
  }
  long long ans = 0;
  for (int i = 0; i <= n; i++) ans += f[i][m], ans %= 1000000007;
  printf("%I64d\n", ans);
}
