#include<bits/stdc++.h>
#define ll long long
using namespace std;
const int N=1e5+5;
char s[N];
int pre[5][N],nxt[5][N];
string ss="spbsu";
void solve(){
	int n;
	scanf("%d",&n);
	scanf("%s",s+1);
	for(int k=0;k<2;k++)
		for(int i=1;i<=n;i++)
			pre[k][i]=s[i]==ss[k]?i:pre[k][i-1];
	for(int k=3;k<5;k++)
		for(int i=n;i>=1;i--)
			nxt[k][i]=s[i]==ss[k]?i:nxt[k][i+1];
	int ans=1e9;
	for(int i2=1;i2<=n;i2++){
		if(s[i2]!='b') continue;
		int i1=pre[1][i2],i0=pre[0][i1];
		int i3=nxt[3][i2],i4=nxt[4][i3];
		if(i1==0||i4==0) continue;
		int res=(i1-i0-1)+(i2-i1-1)*2+(i3-i2-1)*2+(i4-i3-1);
		ans=min(ans,res);
	}
	printf("%d\n",ans);
}
int main(){
	/*int T;
	scanf("%d",&T);
	while(T--) */solve();
}