#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];
#pragma GCC optimize("O3")
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
inline int readChar();
template<class T = int> inline T readInt();
template<class T> inline void writeInt(T x, char end = 0);
inline void writeChar(int x);
inline void writeWord(const char *s);
static const int buf_size = 1 << 18;
inline int getChar(){
#ifndef LOCAL
static char buf[buf_size];
static int len = 0, pos = 0;
if(pos == len) pos = 0, len = fread(buf, 1, buf_size, stdin);
if(pos == len) return -1;
return buf[pos++];
#endif
}
inline int readChar(){
#ifndef LOCAL
int c = getChar();
while(c <= 32) c = getChar();
return c;
#else
char c; cin >> c; return c;
#endif
}
template <class T>
inline T readInt(){
#ifndef LOCAL
int s = 1, c = readChar();
T x = 0;
if(c == '-') s = -1, c = getChar();
while('0' <= c && c <= '9') x = x * 10 + c - '0', c = getChar();
return s == 1 ? x : -x;
#else
T x; cin >> x; return x;
#endif
}
static int write_pos = 0;
static char write_buf[buf_size];
inline void writeChar(int x){
if(write_pos == buf_size) fwrite(write_buf, 1, buf_size, stdout), write_pos = 0;
write_buf[write_pos++] = x;
}
template <class T>
inline void writeInt(T x, char end){
if(x < 0) writeChar('-'), x = -x;
char s[24]; int n = 0;
while(x || !n) s[n++] = '0' + x % 10, x /= 10;
while(n--) writeChar(s[n]);
if(end) writeChar(end);
}
inline void writeWord(const char *s){
while(*s) writeChar(*s++);
}
struct Flusher{
~Flusher(){ if(write_pos) fwrite(write_buf, 1, write_pos, stdout), write_pos = 0; }
}flusher;
ll cost(int i, int j) {
return max(sum[j] - sum[pos[i - 1] - 1] - (ll)(j - pos[i - 1] + 1) * (i - 1), 0ll);
}
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;
}
return lo;
}
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);
N = readInt();
K = readInt();
for(int i = 0; i < 2 * N; ++i) str += readChar();
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];
writeInt(alienTrick(), ' ');
return 0;
}
