#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
i64 n;
string t = "spbsu";
string s;
i64 dp1[100005][6];
i64 last1[100005][6];
i64 dp2[100005][6];
i64 last2[100005][6];
void solve(){
    cin >> n;
    s.clear();
    cin >> s;

    for(int i = 0; i <= n + 1; i++){
    	for(int j = 0; j <= 5; j++){
    		dp1[i][j] = -1;
    		dp2[i][j] = -1;
    		last1[i][j] = -1;
    		last2[i][j] = -1;
    	}
    }
    for (int i = 0; i < n; i++){
    	for(int j = 0; j < 5; j++){
    		if(i > 0){
    			dp1[i][j] = dp1[i-1][j];
    			last1[i][j] = last1[i-1][j];
    		}
        }
        for(int j = 4; j >= 0; j--){
        	if(s[i] != t[j]){
        		continue;
        	}
        	if(j == 0){
        		if(dp1[i][j] == -1){
        			dp1[i][j] = 0;
        			last1[i][j] = i;
        		}
        		else{
        			dp1[i][j] = 0;
        			last1[i][j] = i;
        		}
        	}
        	else{
        		if(dp1[i][j-1] != -1){
        			dp1[i][j] = dp1[i][j-1] + (i - 1 - last1[i][j-1])*(j);
        			last1[i][j] = i;
        		}
        	}
        }
    }
    for(int i = n-1; i >= 0; i--){
    	for(int j = 0; j < 5; j++){
    		if(i < n-1){
    			dp2[i][j] = dp2[i+1][j];
    			last2[i][j] = last2[i+1][j];
    		}
        }
        for(int j = 0; j < 5; j++){
        	if(s[i] != t[j]){
        		continue;
        	}
        	if(j == 4){
        		if(dp2[i][j] == -1){
        			dp2[i][j] = 0;
        			last2[i][j] = i;
        		}
        		else{
        			dp2[i][j] = 0;
        			last2[i][j] = i;
        		}
        	}
        	else{
        		if(dp2[i][j+1] != -1){
        			dp2[i][j] = dp2[i][j+1] + (last2[i][j+1] - i - 1)*(4-j);
        			last2[i][j] = i;
        		}
        	}
        }
    }
    i64 res = 2e9;
    for (int i = 0; i < n; i++){
        for(int j = 0; j < 5; j++){
        	if(s[i] != t[j]){
        		continue;
        	}
        	if(dp1[i][j] != -1 && dp2[i][j] != -1){
        		res = min(res, dp1[i][j] + dp2[i][j]);
        	}
        	//cout << dp1[i][j] << ' ' << dp2[i][j] << ' ' << s[i] << ' ' << t[j] << ' ' << i << endl;
        }
    }
    
    cout << res << endl;
    
    return;
}
int main(){
    std::ios::sync_with_stdio(false);
    std::cin.tie(0);

	long long tt = 1;
	//cin >> tt;
	while(tt--){
		solve();
	}

    return 0;
}