#include <bits/stdc++.h>
#pragma GCC optimize("O3")
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("avx,avx2")
using namespace std;
using lld = long long int;
using pll = pair<long long int, long long int>;
using pii = pair<int, int>;
using pil = pair<int, long long int>;
using pli = pair<long long int, int>;
using piii = pair<pii, int>;
using lf = double;
using pff = pair<lf, lf>;
int n, m;
char a[209];
int d[2][209][209];
int main() {
  int i, j, k, l;
  int t = 1, tv = 0;
  while (t--) {
    scanf("%d %d", &n, &m);
    scanf("%s", a);
    char xx[9];
    scanf("%s", xx);
    if (xx[0] == xx[1]) {
      int cnt = 0;
      for (i = 0; i < n; i++) {
        if (a[i] == xx[0]) {
          cnt++;
        } else {
          if (m > 0) {
            m--;
            cnt++;
          }
        }
      }
      printf("%d\n", cnt * (cnt - 1) / 2);
      continue;
    }
    for (i = 0; i < n; i++) {
      if (a[i] == xx[0]) {
        a[i] = 1;
      } else if (a[i] == xx[1]) {
        a[i] = 2;
      } else {
        a[i] = 0;
      }
    }
    int cur = 0;
    int nxt = 1;
    for (i = 0; i <= m; i++) {
      for (j = 0; j <= n; j++) {
        d[0][i][j] = d[1][i][j] = -1;
      }
    }
    d[0][0][0] = 0;
    for (k = 0; k < n; k++) {
      for (i = 0; i <= m; i++) {
        for (j = 0; j <= n; j++) {
          if (d[cur][i][j] == -1) continue;
          if (a[k] == 0) {
            d[nxt][i][j] = max(d[nxt][i][j], d[cur][i][j]);
            if (i < m) {
              d[nxt][i + 1][j] = max(d[nxt][i + 1][j], d[cur][i][j] + j);
              d[nxt][i + 1][j + 1] = max(d[nxt][i + 1][j + 1], d[cur][i][j]);
            }
          } else if (a[k] == 1) {
            d[nxt][i][j + 1] = max(d[nxt][i][j + 1], d[cur][i][j]);
            if (i < m) {
              d[nxt][i + 1][j] = max(d[nxt][i + 1][j], d[cur][i][j] + j);
            }
          } else if (a[k] == 2) {
            d[nxt][i][j] = max(d[nxt][i][j], d[cur][i][j] + j);
            if (i < m) {
              d[nxt][i + 1][j + 1] = max(d[nxt][i + 1][j + 1], d[cur][i][j]);
            }
          }
        }
      }
      for (i = 0; i <= m; i++) {
        for (j = 0; j <= n; j++) {
          d[cur][i][j] = -1;
        }
      }
      cur ^= 1;
      nxt ^= 1;
    }
    int res = -1;
    for (i = 0; i <= m; i++) {
      for (j = 0; j <= n; j++) {
        if (res < d[cur][i][j]) {
          res = d[cur][i][j];
        }
      }
    }
    printf("%d\n", res);
  }
}
