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

int main() {
	int n;
    cin >> n;

    string track;
    cin >> track;

    int length = track.size();

    int result = 0;

    if (track[0] == 'H') {
        track[0] = 'B';
        result++;
    }

    map<char, int> values = {{'D', 9}, {'U', -11}, {'B', 10}};
    
    int speed = 0;
    int hsAvailable = 0;

    for(int i = 0; i < length; i++) {
        if (track[i] == 'H') {
            hsAvailable++;
        }
        else {
            speed += values[track[i]];

            if (speed <= 0) {
                int bsRequired = (-speed) / 10 + 1;

                if (bsRequired > hsAvailable) {
                    cout << "-1";
                    return 0;
                }

                hsAvailable -= bsRequired;
                result += bsRequired;
                speed += 10 * bsRequired;
            }
        }
    }

    cout << result << endl;
}