#include <bits/stdc++.h>
using namespace std;
string s;
int ans;
const int N = 100001;
int t[3][2][51][51];
const int INF = 1e9;
void cmax(int &x, int y) {
x = max(x, y);
}
int main() {
ios::sync_with_stdio(0);
cin.tie(0);
int n, k;
cin >> n >> k;
if (k < 3) {
cout << 0 << endl;
} else {
cin >> s;
for (int a = 0; a <= k; ++a) {
for (int b = 0; b <= k; ++b) {
t[0][0][a][b] = -INF;
t[1][0][a][b] = -INF;
t[2][0][a][b] = -INF;
t[0][1][a][b] = -INF;
t[1][1][a][b] = -INF;
t[2][1][a][b] = -INF;
}
}
t[0][0][0][0] = 0;
for (int i = 1; i <= n; ++i) {
auto state = [&](int i, int a, int b, int c) -> int& {
if (!a) return t[0][i % 2][b][c];
if (!b) return t[1][i % 2][a][c];
return t[2][i % 2][a][b];
};
for (int a = 0; a <= k; ++a) {
for (int b = 0; b <= k - a; ++b) {
for (int c = 0; c <= k - a - b; ++c) {
if (a && b && b) break;
cmax(state(i, a, b, c), state(i - 1, a, b, c));
}
}
}
for (int a = 0; a <= k; ++a) {
for (int b = 0; b <= k - a; ++b) {
for (int c = 0; c <= k - a - b; ++c) {
if (a && b && c) break;
if (a && b && s[i - 1] == 'C' && a + b + c < k) {
cmax(state(i, a - 1, b - 1, c), state(i - 1, a, b, c) + 1);
}
if (a && c && s[i - 1] == 'B' && a + b + c < k) {
cmax(state(i, a - 1, b, c - 1), state(i - 1, a, b, c) + 1);
}
if (b && c && s[i - 1] == 'A' && a + b + c < k) {
cmax(state(i, a, b - 1, c - 1), state(i - 1, a, b, c) + 1);
}
if (s[i - 1] == 'A' && a + 1 <= k && a + b + c + 1 <= k && (!b || !c)) {
cmax(state(i, a + 1, b, c), state(i - 1, a, b, c));
}
if (s[i - 1] == 'B' && b + 1 <= k && a + b + c + 1 <= k && (!a || !c)) {
cmax(state(i, a, b + 1, c), state(i - 1, a, b, c));
}
if (s[i - 1] == 'C' && c + 1 <= k && a + b + c + 1 <= k && (!a || !b)) {
cmax(state(i, a, b, c + 1), state(i - 1, a, b, c));
}
}
}
}
for (int a = 0; a <= k; ++a) {
for (int b = 0; b <= k - a; ++b) {
for (int c = 0; c <= k - a - b; ++c) {
if (a && b && c) break;
cmax(ans, state(i, a, b, c));
}
}
}
}
cout << ans << endl;
}
}
