#include <bits/stdc++.h>
using namespace std;
const long long maxn = 1e6 + 5;
const long long mod = 1e9 + 7;
char s[maxn], t[maxn];
int n, m;
int dp[205][205][205];
int main() {
  ios::sync_with_stdio(false);
  cin >> n >> m;
  cin >> (s + 1) >> (t + 1);
  if (t[1] == t[2]) {
    int cnt = 0;
    for (int i = 1; i <= n; i++) cnt += (t[1] == s[i]);
    cnt = min(n, cnt + m);
    cout << cnt * (cnt - 1) / 2 << '\n';
    return 0;
  }
  memset(dp, 128, sizeof dp);
  for (int i = 0; i <= m; i++) dp[0][i][0] = 0;
  for (int i = 1; i <= n; i++)
    for (int j = 0; j <= m; j++)
      for (int k = 0; k <= i; k++) {
        dp[i][j][k] = dp[i - 1][j][k];
        if (s[i] == t[1])
          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[2])
          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);
      }
  int ans = 0;
  for (int i = 0; i <= n; i++) ans = max(ans, dp[n][m][i]);
  cout << ans << '\n';
  return 0;
}
