#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1e9 + 7;
const int maxn = 2005;
int n, K;
int dp[maxn][maxn], sum[maxn];
char s[maxn];
int main() {
  scanf("%d%d", &n, &K);
  scanf("%s", s + 1);
  sum[0] = 1;
  dp[0][0] = 1;
  for (int i = 1; i <= n; i++) {
    for (int j = 0; j <= K; j++) {
      for (int k = i - 1; k >= 0 && (i - k) * (n - i + 1) <= j; k--) {
        dp[i][j] = (dp[i][j] + dp[k][j - (i - k) * (n - i + 1)]) % MOD;
      }
      dp[i][j] = (dp[i][j] * (long long)('z' - s[i])) % MOD;
      dp[i][j] = (dp[i][j] + (long long)(s[i] - 'a') * sum[j]) % MOD;
      sum[j] = (sum[j] + dp[i][j]) % MOD;
    }
  }
  cout << sum[K] % MOD << endl;
  return 0;
}
