#include <bits/stdc++.h>
using namespace std;
const int N = 200, INF = (1 << 30) - 1;
int n, m, dp[N + 9][N + 9][N + 9];
char s[N + 9], t[4];
int main() {
  scanf("%d%d%s%s", &n, &m, s + 1, t);
  if (t[0] == t[1]) {
    int cnt = 0;
    for (int i = 1; i <= n; ++i)
      if (s[i] ^ t[0] && m) {
        s[i] = t[0];
        --m;
      }
    for (int i = 1; i <= n; ++i) cnt += s[i] == t[0];
    printf("%d\n", cnt * (cnt - 1) >> 1);
    return 0;
  }
  for (int i = 0; i <= n + 1; ++i)
    for (int j = 0; j <= n + 1; ++j)
      for (int k = 0; k <= m + 1; ++k) dp[i][j][k] = -INF;
  dp[0][0][0] = 0;
  int ans = 0;
  for (int i = 1; i <= n; ++i)
    for (int j = 0; j <= i; ++j)
      for (int k = 0; k <= m; ++k) {
        int t0 = s[i] == t[0], t1 = s[i] == t[1];
        if (j - t0 >= 0) dp[i][j][k] = dp[i - 1][j - t0][k] + t1 * j;
        if (j > 0 && k > 0)
          dp[i][j][k] = max(dp[i][j][k], dp[i - 1][j - 1][k - 1]);
        if (k > 0) dp[i][j][k] = max(dp[i][j][k], dp[i - 1][j][k - 1] + j);
        ans = max(ans, dp[i][j][k]);
      }
  printf("%d\n", ans);
  return 0;
}
