#include <bits/stdc++.h>
using namespace std;
long long gcd(long long a, long long b) { return a % b ? gcd(b, a % b) : b; }
const long long mod = 1e9 + 7;
const long long inf = 0x3f3f3f3f;
long long qpow(long long a, long long b) {
  long long t = 1;
  while (b != 0) {
    if (b & 1) t = (t * a) % mod;
    a = (a * a) % mod;
    b >>= 1;
  }
  return t;
}
string s, t;
int dp[210][210][210];
int n, m;
signed main() {
  cin >> n >> m;
  cin >> s;
  cin >> t;
  if (t[0] == t[1]) {
    int cnt = 0;
    for (auto& c : s)
      if (c == t[0]) ++cnt;
    cnt += m;
    cnt = min(cnt, n);
    cout << cnt * (cnt - 1) / 2 << endl;
    return 0;
  }
  s = '#' + s;
  memset(dp, -inf, sizeof(dp));
  dp[0][0][0] = 0;
  int ans = 0;
  for (int i = 1; i <= n; ++i) {
    for (int j = 0; j <= i && j <= m; ++j) {
      for (int k = 0; k <= i; ++k) {
        dp[i][j][k] = dp[i - 1][j][k];
        if (k) {
          if (s[i] == t[0]) {
            dp[i][j][k] = max(dp[i][j][k], dp[i - 1][j][k - 1]);
          } else if (j) {
            dp[i][j][k] = max(dp[i][j][k], dp[i - 1][j - 1][k - 1]);
          }
        }
        if (s[i] == t[1]) {
          dp[i][j][k] = max(dp[i][j][k], dp[i - 1][j][k] + k);
        } else if (j) {
          dp[i][j][k] = max(dp[i][j][k], dp[i - 1][j - 1][k] + k);
        }
        ans = max(ans, dp[i][j][k]);
      }
    }
  }
  cout << ans << endl;
  return 0;
}
