// NOTE: it is recommended to use this even if you don't understand the following code.

#include <fstream>
#include <iostream>
#include <string>
#include <vector>

using namespace std;

int main() {
    // uncomment the two following lines if you want to read/write from files
    // ifstream cin("input.txt");
    // ofstream cout("output.txt");

    int N;
    cin >> N;

    string S;
    cin >> S;

    int K = 0;
    int Csere = 0;
    int Hdb = 0;


    // INSERT YOUR CODE HERE
    for(int i = 0 ; i < N; ++i){
        if(S[i] == 'D'){
         K+=9;
        
        }
        else{
            if(S[i] == 'U'){
                K-=11;
            }
            else{
                if(S[i] == 'B'){
                    K+=10;
                }
                else{
                    if(S[i] == 'H'){
                        
                            Hdb+=1;
                           
                        
                    }
                }
            }
        }
 
 if(K<1) {
    if(Hdb>0){
        Csere+=1;
        K+=10;
        Hdb-=1;
    }
    else{
        Csere= -1;
    }
 }}
  
        

    cout << Csere << endl;

    return 0;
}