// 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;
    bool rossz = false;
    int i = 0,seb=0,dbh=0;
    while(i<N and not rossz){
        if (S[i]=='H') dbh+=1;
        else if (S[i]=='D') seb+=9;
             else if(S[i]=='B') seb+=10;
                  else if(S[i]=='U') seb-=11;
        if(seb<1) if(dbh==0) rossz=true;
                  else {
                    while(dbh>0 and seb<1)
                    {k+=1; dbh-=1; seb+=10;}
                    if(seb<1) rossz=true;
                  }
        //cout<<"v= "<<seb<<"k= "<<k;
        ++i;
    }
    if(rossz) k=-1;
    cout << k << endl;


    // INSERT YOUR CODE HERE


    
    
    return 0;
}