#include <bits/stdc++.h>
#ifdef LOCAL
#include "cp_debug.h"
#else
#define debug(...) 42
#endif
#define rep(i,a,b) for(int i = a; i < b; ++i)
#define all(x) begin(x), end(x)
#define sz(x) (int)x.size()
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef pair<string, int> Op;

template<typename T> void read(T &x){
    x = 0;char ch = getchar();ll f = 1;
    while(!isdigit(ch)){if(ch == '-')f*=-1;ch=getchar();}
    while(isdigit(ch)){x = x*10+ch-48;ch=getchar();}x*=f;
}
template<typename T, typename... Args> void read(T &first, Args& ... args) {
    read(first);
    read(args...);
}
template<typename T> void write(T x) {
    if(x > 9) write(x/10);
    putchar(x%10 + '0');
}

const int INF = 0x3f3f3f3f;
signed main() {
  int n;
  read(n);
  string s;
  cin >> s;
  vector<vi> nxt(n+1, vi(26, n)), prev(n+1, vi(26, -1));
  for(int i = n-2; i >= 0; i--) {
    nxt[i] = nxt[i+1];
    nxt[i][s[i+1]-'a'] = i+1;
  }
  for(int i = 1; i < n; i++) {
    prev[i] = prev[i-1];
    prev[i][s[i-1]-'a'] = i-1;
  }
  int ans = INF;
  for(int i = 0; i < n; i++) if(s[i] == 'b') {
    int p1 = prev[i]['p'-'a'];
    if(p1 < 0) continue;
    int p2 = prev[p1]['s'-'a'];
    if(p2 < 0) continue;
    int p3 = nxt[i]['s'-'a'];
    if(p3 >= n) continue;
    int p4 = nxt[p3]['u'-'a'];
    if(p4 >= n) continue;
    ans = min(ans, (p2-p1-1) + 2*(i-p2-1) + 2*(p3-i-1) + (p4-p3-1));
  }
  cout << ans << endl;
  return 0;
}