#include <bits/stdc++.h>
using namespace std;
const int maxn = 2010;
const long long mod = 1000000007;
long long dp[maxn][maxn], sum[maxn];
char s[maxn];
int main() {
  int n, k;
  scanf("%d%d", &n, &k);
  scanf("%s", s + 1);
  dp[0][0] = 1;
  sum[0] = 1;
  for (int i = 1; i <= n; i++) {
    for (int j = 0; j <= k; j++) {
      for (int l = i - 1; l >= 0 && j - (n - i + 1) * (i - l) >= 0; l--) {
        dp[i][j] = (dp[i][j] + ((long long)('z' - s[i]) *
                                dp[l][j - (n - i + 1) * (i - l)]) %
                                   mod) %
                   mod;
      }
      dp[i][j] = (dp[i][j] + ((long long)(s[i] - 'a') * sum[j]) % mod) % mod;
      sum[j] = (sum[j] + dp[i][j]) % mod;
    }
  }
  long long ans = 0;
  for (int i = 0; i <= n; i++) {
    ans = (ans + dp[i][k]) % mod;
  }
  printf("%lld\n", ans);
}
