#include <bits/stdc++.h>
using namespace std;
int n, k;
string s;
int f[2222][2222][2];
const int mod = 1000000007;
int sum[2222][2222];
inline void add(int i, int j, int k, int val) {
  f[i][j][k] += val;
  if (f[i][j][k] >= mod) f[i][j][k] -= mod;
}
int solve() {
  int ans = 0;
  for (int pos = 0; pos < n; ++pos) {
    int ls = s[pos] - 'a';
    int mr = 25 - ls;
    for (int cnt = 0; cnt <= k; ++cnt) {
      add(pos, cnt, 0, ((long long)ls * (sum[cnt][pos - 1])) % mod);
      if (!cnt) {
        add(pos, cnt, 0, ls);
      }
      for (int prev = pos - 1; prev >= 0; --prev) {
        int to_add = (pos - prev) * (n - pos);
        if (to_add > cnt) break;
        add(pos, cnt, 1, ((long long)mr * f[prev][cnt - to_add][0]) % mod);
        add(pos, cnt, 1, ((long long)mr * f[prev][cnt - to_add][1]) % mod);
      }
      int to_add = (pos + 1) * (n - pos);
      if (to_add == cnt) {
        add(pos, cnt, 1, mr);
      }
      sum[cnt][pos] += f[pos][cnt][0];
      if (sum[cnt][pos] >= mod) sum[cnt][pos] -= mod;
      sum[cnt][pos] += f[pos][cnt][1];
      if (sum[cnt][pos] >= mod) sum[cnt][pos] -= mod;
      sum[cnt][pos] += sum[cnt][pos - 1];
      if (sum[cnt][pos] >= mod) sum[cnt][pos] -= mod;
    }
    ans += f[pos][k][0];
    if (ans >= mod) ans -= mod;
    ans += f[pos][k][1];
    if (ans >= mod) ans -= mod;
  }
  if (k == 0) {
    ++ans;
    if (ans == mod) ans = 0;
  }
  return ans;
}
int main() {
  cin >> n >> k;
  cin >> s;
  cout << solve() << endl;
}
