#include <bits/stdc++.h>
using namespace std;
char x[1<<17];
vector<int> a[4];
int lb(int idx,int key)
{
    if(a[idx][0]>=key)return -1;
    int lo=0;
    int hi=a[idx].size()-1;
    while(lo<hi)
    {
        int mid=(lo+hi+1)/2;
        if(a[idx][mid]<key)
        {
            lo=mid;
        }
        else hi=mid-1;
    }
    return a[idx][lo];
}
int ub(int idx,int key)
{
    if(a[idx].back()<=key)return -1;
    int lo=0;
    int hi=a[idx].size()-1;
    while(lo<hi)
    {
        int mid=(lo+hi)/2;
        if(a[idx][mid]>key)
        {
            hi=mid;
        }
        else lo=mid+1;
    }
    return a[idx][hi];
}
int main()
{
    int n;cin>>n;
    scanf("%s",x);
    for(int i=0;i<n;i++)
    {
        if(x[i]=='s')a[0].push_back(i);
        if(x[i]=='p')a[1].push_back(i);
        if(x[i]=='b')a[2].push_back(i);
        if(x[i]=='u')a[3].push_back(i);
    }
    int ans=(1<<20);
    for(int i=0;i<a[2].size();i++)
    {
        int t=a[2][i];
        int u1=lb(1,t);
        if(u1==-1)continue;
        int u2=lb(0,u1);
        if(u2==-1)continue;
        int v1=ub(0,t);
        if(v1==-1)continue;
        int v2=ub(3,v1);
        if(v2==-1)continue;
        ans=min(ans,v2+v1-u1-u2-6);
    }
    cout<<ans;
}