#include <bits/stdc++.h>
using namespace std;
string s, p;
long long n, m;
const long long N = 201;
long long dp[N][N][N];
long long go(long long ind, long long left, long long cnt_a) {
  if (ind == n) return 0;
  if (dp[ind][left][cnt_a] != -1) return dp[ind][left][cnt_a];
  long long ans = 0;
  if (left > 0) {
    if (s[ind] != p[0]) ans = go(ind + 1, left - 1, cnt_a + 1);
    if (s[ind] != p[1]) ans = max(ans, cnt_a + go(ind + 1, left - 1, cnt_a));
  }
  if (s[ind] == p[1])
    ans = max(ans, cnt_a + go(ind + 1, left, cnt_a));
  else if (s[ind] == p[0])
    ans = max(ans, go(ind + 1, left, cnt_a + 1));
  else {
    ans = max(ans, go(ind + 1, left, cnt_a));
  }
  return dp[ind][left][cnt_a] = ans;
}
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cin >> n >> m;
  cin >> s >> p;
  if (p[0] == p[1]) {
    long long cnt = 0;
    for (char i : s)
      if (i == p[0]) cnt++;
    cnt = min(n, cnt + m);
    cout << (cnt * (cnt - 1)) / 2;
    return 0;
  }
  memset(dp, -1, sizeof dp);
  cout << go(0, m, 0);
  return 0;
}
