#include <bits/stdc++.h>
#define MAX 1000005
#define MOD (ll)(1048573)
#define INF (ll)(1e12)
#define inf (int)(1e9)
#define eps (double)(1e-8)
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
int tt, N, K;
string str;
int cnt[MAX], pos[MAX], can[MAX], q[MAX], k[MAX];
ll dp[MAX], sum[MAX];
ll cost(int i, int j) {
return max(sum[j] - sum[pos[i - 1] - 1] - (ll)(j - pos[i - 1] + 1) * (i - 1), -INF);
}
ll func(int i, int j) {
return dp[i] + cost(i + 1, j);
}
int cross(int i, int j) {
// int lo = j , hi = N + 1;
// while(lo + 1 < hi) {
// int mid = (lo + hi) >> 1;
// if(func(i, mid) <= func(j, mid)) lo = mid;
// else hi = mid;
// }
//sum[j] - j * (i - 1) - sum[pos[i - 1] - 1] + (ll)(pos[i - 1] - 1) * (i - 1)
//x * (j - i) + C1 <= C2
ll C1 = dp[i] - sum[pos[i] - 1] + (ll)(pos[i] - 1) * (i), C2 = dp[j] - sum[pos[j] - 1] + (ll)(pos[j] - 1) * (j);
ll x1 = min((C2 - C1) / (j - i), (ll)N);
//cout << i << ' ' << j << ' ' << x1 << ' ' << lo << endl;
return x1;
}
void monotoneQueue(ll lambda) {
int str = 0, ed = 1;
q[0] = 0; can[0] = N;
for(int i = 1; i <= N; ++i) {
while(can[str] < i) ++str;
dp[i] = func(q[str], i) + lambda; k[i] = k[q[str]] + 1;
while(str + 1 < ed && can[ed - 2] >= cross(q[ed - 1], i)) --ed;
can[ed - 1] = cross(q[ed - 1], i); q[ed] = i; can[ed] = N;
++ed;
}
return;
}
ll alienTrick() {
ll lo = -1, hi = INF;
ll ans = 0;
while(lo + 1 < hi) {
ll mid = (lo + hi) >> 1;
monotoneQueue(mid);
ans = max(ans, dp[N] - mid * K);
if(k[N] >= K) lo = mid;
else hi = mid;
}
return ans;
}
int main() {
ios::sync_with_stdio(0);
cin.tie(NULL);
cin >> N >> K >> str;
for(int i = 0, tmp = 0, p = 0; i < 2 * N; ++i) {
if(str[i] == 'A') cnt[++p] = tmp;
else ++tmp;
}
for(int i = 0, j = 1; i <= N; ++i) {
while(j <= N && cnt[j] < i) ++j;
pos[i] = max(i + 1, j);
}
for(int i = 1; i <= N; ++i) sum[i] = sum[i - 1] + cnt[i];
// for(int i = 0; i <= N; ++i) {
// cout << - sum[pos[i - 1] - 1] + (ll)(pos[i - 1] - 1) * (i - 1) << endl;
// }
// for(int i = 1; i <= N; ++i) for(int j = i; j <= N; ++j) {
// cout << i << ' ' << j << ' ' << cost(i, j) << endl;
// }
cout << alienTrick();
return 0;
}
