#include <bits/stdc++.h>
using namespace std;
const int MAXN = 2030, Mod = 1000000007;
int n, k;
char s[MAXN];
long long f[MAXN][MAXN], ret = 0, sum[MAXN];
int main() {
  scanf("%d%d", &n, &k);
  scanf("%s", s + 1);
  f[0][0] = 1;
  sum[0] = 1;
  if (k == 0) ret += f[0][0];
  for (int i = 1; i <= n; ++i) {
    for (int j = 0; j <= k; ++j) {
      f[i][j] = sum[j] * (s[i] - 'a') % Mod;
      for (int pre = 0; pre < i && (pre + 1) * (n - i + 1) <= j; ++pre)
        f[i][j] = (f[i][j] + f[i - pre - 1][j - (pre + 1) * (n - i + 1)] *
                                 ('z' - s[i]) % Mod) %
                  Mod;
      sum[j] = (sum[j] + f[i][j]) % Mod;
    }
    ret = (ret + f[i][k]) % Mod;
  }
  printf("%d\n", int(ret));
  return 0;
}
