#include <bits/stdc++.h>
const int maxn = 2001, maxm = 2001, mod = 1000000007;
int n, m, f[maxn][maxm];
char s[maxn];
void inc(int &x, int y) {
  x += y;
  if (x >= mod) x -= mod;
}
int main() {
  scanf("%d%d%s", &n, &m, s + 1);
  f[0][0] = 1;
  for (int i = 1; i <= n; ++i) {
    s[i] -= 'a' - 1;
    for (int j = 0; j <= m; ++j) {
      f[i][j] = (long long)f[i - 1][j] * s[i] % mod;
      for (int k = 1; k <= i && k * (n - i + 1) <= j; ++k)
        inc(f[i][j],
            (long long)(f[i - k][j - k * (n - i + 1)] -
                        (k < i ? f[i - k - 1][j - k * (n - i + 1)] : 0) + mod) *
                (26 - s[i]) % mod);
    }
  }
  printf("%d\n", f[n][m]);
  return 0;
}
