#include<bits/stdc++.h>
#include<iostream>
#include<string>
#include<cmath>
#include<cstdio>
#include<cctype>
#include<cstring>
#include<iomanip>
#include<cstdlib>
#include<ctime>
#include<set>
#include<map>
#include<utility>
#include<queue>
#include<vector>
#include<stack>
#include<sstream>
#include<algorithm>
/************************************************/
#define int long long
#define rep(i,n) for(int i=0;i<n;i++)
#define m_p make_pair
#define pb push_back
#define fr first
#define se second
#define ford(i,n) for(int i=n-1;i>=0;i--)
#define forn(i,a,n) for(int i=a;i<n;i++)
#define foreach(i,c) for(__typeof(c.begin())i=(c.begin());i!=(c).end();i++)
#define pii pair<int,int>
#define vi vector<int>
#define ll long long
#define vll vector<ll>
#define sz(s) (int)(s.size())
#define all(s) s.begin(),s.end()
#define zero(x) memset(x,0,sizeof(x))
#define vii vector<pair<int,int> >
#define mpis map<int,string>
#define mpii map<int,int>
#define mpsi map<string,int>
#define re return
#define mod 998244353
/************************************************/
using namespace std;

long long get(){
	char c=getchar();
	long long x=0LL;
	while(c<'0'||c>'9')
	c=getchar();
	while(c>='0'&&c<='9'){
		x*=10LL;
		x+=(c-'0');
		c=getchar();
	}
	return x;
}
string i_t_s(int x){
	string t="";
	while(x){
		t+=x%10+'0';
		x/=10;
	}
	reverse(all(t));
	re t;
}
int s_t_i(string t){
	int x=0;
	rep(i,sz(t)){
		x=x*10+(t[i]-'0');
	}
	re x;
}
ll q_p(ll x,ll y){
	ll res=1;
	x%=mod;
	while(y){
		if(y%2){
			res=res*x;
			res%=mod;
		}
		y/=2;
		x=x*x;
		x%=mod;
	}
	re res;
}
bool ok(int x,int y,int n,int m){
	 re(x>=0&&x<n&&y>=0&&y<m);
}
bool isprime(int x){
	if(x<2)
	re false;
	for(int i=2;i*i<=x;i++)
	if(x%i==0)
	re false;
	re true;
}
int dx[4]={0,-1,0,1},dy[4]={1,0,-1,0};
int n;
//spbsu
string s;
int q[6]={0,1,2,2,1,0};
int dp[100001][6]; 
signed main(){
	ios::sync_with_stdio(0);
	cin>>n>>s;
	memset(dp,0x3f,sizeof(dp));
	dp[0][0]=0;
	rep(i,n){
		rep(j,6){
			dp[i+1][j]=dp[i][j]+q[j]; 
		}
		if(s[i]=='s'){
			dp[i+1][1]=min(dp[i+1][1],dp[i][0]);
			dp[i+1][4]=min(dp[i+1][4],dp[i][3]);
		}
		if(s[i]=='p'){
			dp[i+1][2]=min(dp[i+1][2],dp[i][1]);
		}
		if(s[i]=='b'){
			dp[i+1][3]=min(dp[i+1][3],dp[i][2]);
		}
		if(s[i]=='u'){
			dp[i+1][5]=min(dp[i+1][5],dp[i][4]);
		}
	}
	cout<<dp[n][5];
	re 0;
}
/*
检查循环是rep(i,n)还是rep(i,m)!!
long long所要的时间比int长!!
没有long long必要不写long long!!
写公式前先想一想!!
二分前看一看边界对不对，有没有特殊值!!
提交前要测特殊数据（边界上的）!!
*/