#include <bits/stdc++.h>
using namespace std;
const int inf = 1e9;
int main() {
cin.tie(nullptr)->sync_with_stdio(false);
int n, k;
string s;
cin >> n >> k >> s;
reverse(s.begin(), s.end());
int u = 0;
for (int i = 0; i < 2 * n; ++i) {
u = (u << 1) | (s[i] == 'B');
}
vector<int> dist(1 << (2 * n), inf);
queue<int> q;
q.push(u);
dist[u] = 0;
while (!q.empty()) {
int u = q.front();
q.pop();
for (int i = 0; i < 2 * n - 1; ++i) {
int nu = u;
int b1 = nu & 1 << i;
int b2 = nu & 1 << (i + 1);
if (b1) { nu ^= 1 << i; }
if (b2) { nu ^= 1 << (i + 1); }
if (b2) { nu ^= 1 << i; }
if (b1) { nu ^= 1 << (i + 1); }
if (dist[u] + 1 < dist[nu]) {
q.push(nu);
dist[nu] = dist[u] + 1;
}
}
}
auto good = [&](int mask) {
vector<int> a, b;
for (int i = 0; i < 2 * n; ++i) {
if (mask & 1 << i) {
b.push_back(i);
} else {
a.push_back(i);
}
}
if (a.size() != b.size()) { return false; }
vector<int> dp(n, inf);
dp[0] = 1;
for (int i = 1; i < n; ++i) {
if (a[i] < b[0]) {
dp[i] = 1;
}
for (int j = i - 1; j >= 0; --j) {
if (a[i] < b[j + 1]) {
dp[i] = min(dp[i], 1 + dp[j]);
}
}
}
return k >= dp[n - 1];
};
int ans = inf;
for (int mask = 0; mask < 1 << (2 * n); ++mask) {
if (good(mask)) {
ans = min(ans, dist[mask]);
}
}
cout << ans << '\n';
}
