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

const int inf = 1'000'000;
const string m = "spbsu";
const int p[5] = {0,4,3,2,1}, q[5] = {0,1,2,3,4};

void solve(int t){
    int n;
    cin>>n;
    
    string s;
    cin>>s;
    
    vector dp(n+1,array<int,6>());
    
    for(int i=n;i>=0;i--){
        for(int j=0;j<6;j++){
            int val = inf;
            
            if(i==n){
                val = (j==5 ? 0 : inf);
            }else{
                if(j==5)val=0;
                else{
                    if(s[i]!=m[j]){
                        val = dp[i+1][j] + min(p[j],q[j]);
                    }else{
                        val = min(dp[i+1][j] + min(p[j],q[j]), dp[i+1][j+1]);
                    }
                }
            }
            
            dp[i][j] = val;
        }
    }
    
    cout<<dp[0][0]<<"\n";
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    
    int t=1;
    //cin>>t;
    for(int i=1;i<=t;i++)solve(i);
    
    return 0;
}