#include <iostream>
#include <vector>
#include <cstdint>

using namespace std;

typedef int64_t ll;

int main() {

    ll n;
    cin >> n;
    vector<ll> v(n, 0);

    for(ll i = 0; i < n; i++) {

        char c;
        cin >> c;
        if(c == 'U') {
            v[i] = -11;
        }else if(c == 'D') {
            v[i] = 9;
        }else if(c == 'H') {
            v[i] = 0;
        }else if(c == 'B') {
            v[i] = 10;
        }

    }

    ll z = 0;
    ll ans = 0;
    
    ll curr = 0;
    
    for(ll i = 0; i < n; i++) {

        curr += v[i];
        if(v[i] == 0) {
            z++;
        }

        if(curr <= 0) {

            ll ned = abs(curr) + 1;

            ll c = ned/10;
            if(ned % 10 != 0) {
                c++;
            }

            if(c <= z) {
                z -= c;
                curr += c * 10;
                ans += c;
            }else{
                cout << "-1\n";
            }

        }

    }

    cout << ans;

}