#include <bits/stdc++.h>
using namespace std;

const int INF = 1e9;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int N;
    cin >> N;
    string S;
    cin >> S;

    // dp[i][v] = min csere, i szakasz után v sebességgel
    // Max sebesség becslés: 100 * 10 = 1000 elég
    const int MAXV = 1000;
    vector<vector<int>> dp(N + 1, vector<int>(MAXV + 1, INF));

    dp[0][0] = 0;

    for (int i = 0; i < N; i++) {
        for (int v = 0; v <= MAXV; v++) {
            if (dp[i][v] == INF) continue;

            char c = S[i];
            if (c == 'D') {
                int nv = v + 9;
                if (nv > 0 && nv <= MAXV)
                    dp[i + 1][nv] = min(dp[i + 1][nv], dp[i][v]);
            }
            else if (c == 'B') {
                int nv = v + 10;
                if (nv > 0 && nv <= MAXV)
                    dp[i + 1][nv] = min(dp[i + 1][nv], dp[i][v]);
            }
            else if (c == 'U') {
                int nv = v - 11;
                if (nv > 0)
                    dp[i + 1][nv] = min(dp[i + 1][nv], dp[i][v]);
            }
            else if (c == 'H') {
                // H marad
                if (v > 0)
                    dp[i + 1][v] = min(dp[i + 1][v], dp[i][v]);

                // H -> B csere
                int nv = v + 10;
                if (nv > 0 && nv <= MAXV)
                    dp[i + 1][nv] = min(dp[i + 1][nv], dp[i][v] + 1);
            }
        }
    }

    int ans = INF;
    for (int v = 1; v <= MAXV; v++) {
        ans = min(ans, dp[N][v]);
    }

    if (ans == INF) cout << -1 << "\n";
    else cout << ans << "\n";

    return 0;
}