#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = (int)1e6 + 10, MOD = 998244353, inf = (int)1e9;
#define int ll
int n, k, a[N], b[N], px[N * 2], py[N * 2], f[N * 2];
string s;
ll dop = 0;
void make() {
int bal = 0;
string ret;
for(int i = 0, col = 0; i < n + n; ++i) {
if(s[i] == 'A') {
ret += s[i];
dop += col;
bal++;
} else {
if(bal) {
bal--;
ret += s[i];
} else {
col++;
}
}
while(col && bal) {
ret += 'B';
bal--;
col--;
}
}
s = ret;
}
ll dp[N], col[N], pr[N];
int nxt[N];
struct line{
ll k, b;
int id;
ll get(ll x) {
return k * x + b;
}
};
deque<line> st;
long double cross(line x, line y) {
return ((x.b - y.b) * 1.0) / ((y.k - x.k) * 1.0);
}
void add(line nv) {
while((int)st.size() >= 2) {
int m = (int)st.size();
if(cross(st[m - 1], st[m - 2]) >= cross(st[m - 2], nv)) {
st.pop_back();
} else {
break;
}
}
st.push_back(nv);
}
pair<ll, int> calc(ll x) {
while((int)st.size() > 1 && cross(st[0], st[1]) < x * 1.0) st.pop_front();
ll ret = st[0].get(x);
ll _ = col[st[0].id] + 1;
return {ret, _};
}
line w[N];
pair<ll, int> solve(ll pen) {
st.clear();
dp[0] = 0;
col[0] = 0;
for(int i = 1; i <= n; i++) {
dp[i] = (ll)1e18;
col[i] = 0;
}
w[0].k=0;w[0].b=0;w[0].id=0;
int it = 0;
for(int r = 1; r <= n; r++) {
while(nxt[it] <= r) {
add(w[it]);
it++;
}
auto [d, c] = calc(r);
dp[r] = d + pr[r] + pen;col[r] = c;
w[r].k = -r;w[r].b = dp[r] - pr[nxt[r] - 1] + r * (nxt[r] - 1);
w[r].id = r;
}
return {dp[n], col[n]};
}
void test() {
cin >> n >> k;
cin >> s;
make();
int fx = 1, fy = 1;
for(int i = 1; i <= n + n; i++) {
if(s[i - 1] == 'A') {
a[fx++] = i;
} else {
b[fy++] = i;
f[i]++;
}
}
for(int i = 1; i <= n + n; i++) {
f[i] += f[i - 1];
}
for(int i = 1; i <= n; i++) {
pr[i] = pr[i - 1] + f[a[i]];
}
for(int i = 0; i <= n; i++) {
if(i) {
nxt[i] = max(i + 1, nxt[i - 1]);
} else {
nxt[i] = i + 1;
}
while(nxt[i] <= n && f[a[nxt[i]]] < i) {
nxt[i]++;
}
}
// auto [x, y] = solve(0);
// cout << x << ' ' << y;
// return;
ll l = -1, r = (ll)1e14, res;
while(r - l > 1) {
ll mid = (l + r) >> 1;
auto [val, t] = solve(mid);
if(t <= k) {
r = mid;
res = val - k * 1ll * mid;
} else {
l = mid;
}
}
cout << res + dop << '\n';
}
int32_t main() {
ios_base::sync_with_stdio(false);
cin.tie(0);
int t = 1;
// cin >> t;
while(t--) {
test();
}
}
