#include <bits/stdc++.h>
using i64 = long long;
#ifdef DEBUG
#include "debug.h"
#else
#define debug(...) void(23)
#endif
template<typename T>
bool chmin(T& a, T b) {
if (a > b) {
a = b;
return true;
}
return false;
}
constexpr int max_N = 5000 + 5;
constexpr i64 inf = i64(1E18) + 5;
int N;
int K;
std::string S;
int main() {
std::ios_base::sync_with_stdio(false);
std::cin.tie(nullptr);
std::cin >> N >> K >> S;
std::vector<int> v[2];
for (int i = 0; i < 2 * N; ++i) {
v[S[i] - 'A'].emplace_back(i);
}
auto calc = [&](i64 mid) {
std::vector<std::pair<i64, int>> f(N + 1, {inf, 0});
f[0] = {0, 0};
for (int i = 0; i < N; ++i) {
int cost = 0;
int p = 0;
for (int j = 1; i + j <= N; ++j) {
while (p < N - i && v[1][i + p] < v[0][i + j - 1]) {
p += 1;
}
cost += p;
chmin(f[i + j], {f[i].first + cost + mid, f[i].second + 1});
}
}
return f[N];
};
i64 lo = 0, hi = 1LL * N * N;
while (lo < hi) {
i64 mid = (lo + hi) >> 1;
if (calc(mid).second <= K) {
hi = mid;
} else {
lo = mid + 1;
}
}
i64 ans = calc(lo).first - 1LL * lo * K;
std::cout << ans << '\n';
return 0;
}
