#include <bits/stdc++.h>
#define rep(i,l,r) for(int i=l; i<=r; i++)
#define IOS {cin.tie(0);cout.tie(0);ios::sync_with_stdio(0);}
using namespace std;

char s[100010];
int lst[100010][26], nxt[100010][26];

typedef long long ll;
typedef pair<int,int> P;

int main() {
    int n; scanf("%d", &n);
    scanf("%s", s+1);
    rep (i, 1, n) {
        rep (j, 0, 25) {
            lst[i][j] = lst[i-1][j];
        }
        lst[i][s[i] - 'a'] = i;
    }
    for (int i=n; i>=1; i--) {
        rep (j, 0, 25) {
            nxt[i][j] = nxt[i+1][j];
        }
        nxt[i][s[i] - 'a'] = i;
    }
    int ans = 1e9;
    rep (i, 1, n) {
        if (s[i] == 'b') {
            int t = lst[i]['p'-'a'];
            int s = lst[t]['s' - 'a'];
            int k = nxt[i]['s' - 'a'];
            int l = nxt[k]['u' - 'a'];
            if (t&&s&&k&&l) ans = min(ans, i - t - 1 + i - s - 2 + k-i-1 + l-i-2);
        }
    }
    cout << ans;
    return 0;
}