#include <bits/stdc++.h>
using namespace std;
const long long inf = -(long long)1e18;
void solve() {
  long long n, k;
  cin >> n >> k;
  string s, t;
  cin >> s >> t;
  vector<vector<vector<long long>>> dp(
      n + 1, vector<vector<long long>>(n + 1, vector<long long>(k + 1, -1)));
  function<long long(long long, long long, long long)> dfs =
      [&](long long idx, long long cnta, long long used) -> long long {
    if (idx == n) {
      return 0;
    }
    long long &an = dp[idx][cnta][used];
    if (an != -1) {
      return an;
    }
    an = 0;
    if (t[0] != t[1]) {
      for (long long j = 0; j < 2; j++) {
        if (s[idx] == t[j]) {
          an = max(an, dfs(idx + 1, cnta + (j == 0), used) + ((j == 1) * cnta));
        } else {
          an = max(an, dfs(idx + 1, cnta, used));
          if (used + 1 <= k) {
            an = max(an, dfs(idx + 1, cnta + (j == 0), used + 1) +
                             ((j == 1) * cnta));
          }
        }
      }
    } else {
      if (s[idx] == t[0]) {
        an = max(an, dfs(idx + 1, cnta + 1, used) + cnta);
      } else {
        an = max(an, dfs(idx + 1, cnta, used));
        if (used + 1 <= k) {
          an = max(an, dfs(idx + 1, cnta + 1, used + 1) + cnta);
        }
      }
    }
    return an;
  };
  cout << dfs(0, 0, 0) << "\n";
}
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  solve();
}
