#include <bits/stdc++.h>
using namespace std;
struct line {
long long k, n;
int f;
void init() {
k = 0;
n = (long long) 1e18;
f = 0;
}
long long val(long long x) {
return k * x + n;
}
};
const int MAX = 5e6;
const long long inf = (long long) 1e18;
int n, ia[MAX], ib[MAX];
long long pref[MAX];
line li[MAX];
int ls[MAX], rs[MAX], tsz, root;
vector<int> a, b;
pair<long long, int> dp[MAX];
void Update(int& x, int l, int r, line ln) {
if (!x) {
x = ++tsz;
li[x].init();
}
int mid = l + r >> 1;
if (li[x].val(mid) > ln.val(mid) || (ln.val(mid) == li[x].val(mid) && ln.f < li[x].f)) {
swap(li[x], ln);
}
if (l == r) {
return;
}
if (ln.val(l) < li[x].val(l)) {
Update(ls[x], l, mid, ln);
} else {
Update(rs[x], mid + 1, r, ln);
}
}
pair<long long, int> Query(int x, int l, int r, int i) {
if (!x) {
return make_pair((long long) 1e18, 0);
}
if (l == r) {
return make_pair(li[x].val(i), li[x].f);
}
int mid = l + r >> 1;
if (i <= mid) {
return min(Query(ls[x], l, mid, i), make_pair(li[x].val(i), li[x].f));
} else {
return min(Query(rs[x], mid + 1, r, i), make_pair(li[x].val(i), li[x].f));
}
}
void clear_seg() {
while (tsz > 0) {
ls[tsz] = 0;
rs[tsz] = 0;
li[tsz].init();
tsz -= 1;
}
root = 0;
}
pair<long long, int> Solve(long long mid) {
for (int i = 1; i <= n; i++) {
dp[i] = make_pair(inf, 0);
}
dp[0] = {0, 0};
int ptr = -1;
multiset<pair<long long, int>> st;
clear_seg();
for (int i = 0; i < n; i++) {
st.insert(dp[i]);
while (ptr + 1 <= i && ia[a[i]] >= ib[b[ptr + 1]]) {
ptr += 1;
line cur;
cur.k = -ptr;
cur.n = dp[ptr].first + (ptr == 0 ? 0 : pref[ptr - 1]) + mid;
cur.f = dp[ptr].second;
Update(root, 0, n, cur);
st.erase(st.find(dp[ptr]));
}
if (!st.empty()) {
pair<long long, int> val = *st.begin();
dp[i + 1] = min(dp[i + 1], {val.first + mid, val.second + 1});
}
if (ptr != -1) {
pair<long long, int> val = Query(root, 0, n, ia[a[i]]);
dp[i + 1] = min(dp[i + 1], {val.first + ia[a[i]] * 1LL * (ptr + 1) - pref[ptr], val.second + 1});
}
}
return dp[n];
}
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
int k;
cin >> n >> k;
string s;
cin >> s;
for (int i = 0; i < 2 * n; i++) {
if (s[i] == 'A') {
a.push_back(i);
} else {
b.push_back(i);
}
}
for (int i = 0; i < n; i++) {
ia[a[i]] = i + 1;
}
for (int i = 0; i < 2 * n; i++) {
if (i > 0) {
pref[i] = pref[i - 1];
}
pref[i] += (s[i] == 'A' ? 1 : 0);
}
for (int i = 0; i < n; i++) {
ib[b[i]] = pref[b[i]];
}
for (int i = 0; i < n; i++) {
if (i > 0) {
pref[i] = pref[i - 1];
} else {
pref[i] = 0;
}
pref[i] += ib[b[i]];
}
long long low = 0, high = 5e11, p = 0;
while (low <= high) {
long long mid = low + high >> 1;
pair<long long, int> res = Solve(mid);
if (res.second <= k) {
p = mid;
high = mid - 1;
} else {
low = mid + 1;
}
}
pair<long long, int> res = Solve(p);
cout << res.first - p * k << '\n';
return 0;
}
