//It's MyGO!!!
//All OK
#include <bits/stdc++.h>
#define ios ios::sync_with_stdio(0);cin.tie(nullptr);cout.tie(0);cout << fixed << setprecision(6)
#define fi first
#define se second
#define INF 0x3f3f3f3f
#define LNF (1ll << 60)
#define ll long long
#define ull unsighed long long
#define int long long
// #define mod 998244353
#define mod 1000000007
using namespace std;

const int N = 10;
ll f[N];

void solve(){
    int n; cin >> n;
    string s; cin >> s;
    string t = " spbsu";
    s = " " + s;
    memset(f, 0x3f, sizeof f);
    f[0] = 0;
    for(int i = 1; i <= n; i ++){
        for(int j = 5; j >= 1; j --){
            if(s[i] == t[j]){
               if(j <= 2)
                    f[j] = min(f[j], f[j - 1] - i);
                else if(j == 3)
                    f[j] = min(f[j], f[j - 1]);
                else
                    f[j] = min(f[j], f[j - 1] + i); 
            }
        }
    }
    cout << f[5] - 6;
}

signed main(){
    ios;

    // init();
    int _ = 1;
    // cin >> _;
    for(int __ = 1; __ <= _; __ ++){
        // cout << "Case #" << __ << ": ";

        solve();
    }
    return 0;
}