#include "bits/stdc++.h"

using namespace std;
using lint = int64_t;
const int inf = 1e9;

int main() {
    cin.tie(nullptr)->sync_with_stdio(false);
    int n;
    string s;
    cin>>n>>s;
    vector<int>dp(n, inf);
    string t = "spbsu";
    for(int i=0;i<n;i++) if(s[i] == t[0]) dp[i] = -i;
    for(int j=1;j<5;j++){
        int mn = inf;
        vector<int>ndp(n, inf);
        for(int i=0;i<n;i++){
            mn = min(mn, dp[i]);
            if(s[i] == t[j]){
                int cnt = 0;
                if(j == 1) cnt = -i;
                if(j == 3 || j == 4) cnt = i;
                ndp[i] = mn + cnt;
            }
        }
        swap(dp, ndp);
    }
    cout<< *min_element(dp.begin(), dp.end()) - 6<<"\n";  

    return 0;
}