#include <bits/stdc++.h>
using namespace std;
const long long mod = 1e9 + 7;
const int N = 2005;
char ch[N];
int n, s, a[N], b[N];
long long f[N][N], g[N];
int main() {
  cin >> n >> s >> (ch + 1);
  for (register int i = 1; i <= n; i++)
    a[i] = ch[i] - 'a', b[i] = 26 - ch[i] + 'a' - 1;
  f[n + 1][0] = 1;
  for (register int i = n; i >= 1; i--)
    for (register int j = 0; j <= s; j++) {
      f[i][j] = a[i] * f[i + 1][j] % mod;
      if ((n - i + 1) <= j)
        f[i][j] = (f[i][j] + f[i + 1][j - (n - i + 1)] * b[i]) % mod;
      int last = n + 1;
      for (register int k = i + 1; k <= n; k++) {
        if (j >= (k - i + 1) * (n - k + 1))
          f[i][j] =
              (f[i][j] + f[k + 1][j - (k - i + 1) * (n - k + 1)] * b[k]) % mod;
        else {
          last = i + 2;
          break;
        }
      }
      for (register int k = n; k >= last; k--) {
        if (j >= (k - i + 1) * (n - k + 1))
          f[i][j] =
              (f[i][j] + f[k + 1][j - (k - i + 1) * (n - k + 1)] * b[k]) % mod;
        else
          break;
      }
      f[i][j] += g[j];
      if (j == 0) f[i][j]++;
      f[i][j] %= mod;
      g[j] += f[i + 1][j] * a[i];
      g[j] %= mod;
    }
  cout << f[1][s] << endl;
}
