#include <bits/stdc++.h>

int32_t main() {
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(nullptr);

    int n, k;
    std::string s;
    std::cin >> n >> k >> s;

    int mask_s = 0;
    for (int i = 0; i < 2 * n; i++) {
        mask_s |= int(s[i] == 'A') << i;
    }
    std::vector<int> dist(1 << 2 * n, 1e9);
    dist[mask_s] = 0;
    for (int i = (1 << 2 * n) - 1; i >= 0; i--) {
        if (dist[i] >= 1e8) {
            continue;
        }
        for (int j = 0; j + 1 < 2 * n; j++) {
            if (!(i >> j & 1) && (i >> j + 1 & 1)) {
                dist[i ^ 3 << j] = std::min(dist[i ^ 3 << j], dist[i] + 1);
            }
        }
    }

    int ans = 1e9;
    for (int i = 0; i < (1 << 2 * n); i++) {
        if (dist[i] < 1e8) {
            std::string s(2 * n, '-');
            for (int j = 0; j < 2 * n; j++) {
                s[j] = "BA"[i >> j & 1];
            }
            int cnt = 0;
            while (s.size()) {
                cnt++;
                int it = std::find(s.begin(), s.end(), 'B') - s.begin();
                if (it == 0) {
                    cnt = 1e9;
                    break;
                }
                s.erase(s.begin(), s.begin() + it);
                for (auto& ch : s) {
                    if (it && ch == 'B') {
                        ch = '-';
                        it--;
                    }
                }
                std::erase(s, '-');
            }
            if (cnt <= k) {
                ans = std::min(ans, dist[i]);
            }
        }
    }

    std::cout << ans << "\n";

    return 0;
}