#include <cstdio>
#include <iostream>
#include <algorithm>
#include <queue>
#include <cmath>
#include <cstring>
#include <string>
#include <stack>
#include <deque>
#include <map>
#include <set>
#include <bitset>
#include <unordered_map>
#include <unordered_set>
using namespace std;
#define ll long long
#define endl "\n"
#define S second
#define F first
#define ln cout<<endl;
#define mem(a) memset((a),0,sizeof (a));
#define yes cout<<"YES"<<endl;
#define no cout<<"NO"<<endl;
#define debug cout<<"here!"<<endl;

ll cnt,n,m,t,ans,ant;
const int N=2e5+10;
const int INF=0x3f3f3f3f;
string str;

void solve()
{
    cin>>n;
    cin>>str;
    vector<ll>s1;
    vector<ll>p1;
    vector<ll>b1;
    vector<ll>u1;

    for(ll i=0;i<n;i++)
    {
        if(str[i]=='s') s1.push_back(i);
        else if(str[i]=='b') b1.push_back(i);
        else if(str[i]=='u') u1.push_back(i);
        else if(str[i]=='p') p1.push_back(i);
    }

    ans=1e18;
    for(ll b:b1)
    {
        ll s,s2,u,p;
        p=lower_bound(p1.begin(),p1.end(),b)-p1.begin();
        if(p==0||p1[p-1]>=b) continue;
        
        p=p1[p-1];
        s=lower_bound(s1.begin(),s1.end(),p)-s1.begin();
        if(s==0||s1[s-1]>=p) continue;
        s=s1[s-1];
        

        s2=lower_bound(s1.begin(),s1.end(),b)-s1.begin();
        if(s2>=s1.size()) continue;
        s2=s1[s2];
        u=lower_bound(u1.begin(),u1.end(),s2)-u1.begin();
        if(u>=u1.size()) continue;
        u=u1[u];
        cnt=(b-2-s) + (b-1-p) + (s2-b-1) + (u-b-2);
        ans=min(ans,cnt);
    }
    cout<<ans<<endl;
    return;
}

int main()
{
    ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
    ll _=1;
    // cin>>_;
    while(_--)
        solve();
    return 0;
}