#include<bits/stdc++.h>
#define int long long
using namespace std;
int n;
char s[100010];
char t[6];
int dp[6];
signed main(){
	memset(dp,0x3f,sizeof(dp));
	scanf("%lld",&n);
	scanf("%s",s + 1);
	t[0] = 's',t[1] = 'p',t[2] = 'b',t[3] = 's',t[4] = 'u';
	dp[0] = 0;
	for(int i = 1; i <= n; i++){
		for(int j = 4; j >= 0; j--){
			if(s[i] == t[j]){
				if(j < 2) dp[j + 1] = min(dp[j + 1],dp[j] - i);
				else if(j > 2) dp[j + 1] = min(dp[j + 1],dp[j] + i);
				else dp[j + 1] = min(dp[j + 1],dp[j]);
			}
		}
	}
	printf("%lld",dp[5] - 6);
	return 0;
}