#include <bits/stdc++.h>
using namespace std;
int n, k;
long long dp[2007][2007], dif[2007][2007];
string s;
const long long mod = 1e9 + 7;
int main() {
  ios_base::sync_with_stdio(0);
  cin >> n >> k;
  cin >> s;
  dp[0][0] = 1;
  dif[0][0] = 1;
  for (int i = 1; i <= n; i++) {
    for (int j = 0; j <= k; j++) {
      dp[i][j] = (dp[i][j] + dp[i - 1][j] * 1LL * (s[i - 1] - 'a' + 1)) % mod;
      for (int p = i; p >= 1; p--) {
        long long sum = 1LL * j + 1LL * (i - p + 1) * (n - i + 1);
        if (sum > k) break;
        dp[i][sum] =
            (dp[i][sum] + 1LL * dif[p - 1][j] * ('z' - s[i - 1])) % mod;
      }
      dif[i][j] = (dp[i][j] - dp[i - 1][j] + mod) % mod;
    }
  }
  cout << dp[n][k];
}
