#include <bits/stdc++.h>

using namespace std;

const int maxn = 1e5 + 5;
const string t = "spbsu";

int n;
int f[maxn][6];
char s[maxn];

int main(){
	scanf("%d", &n);
	scanf("%s", s);
	
	memset(f, 0x3f, sizeof(f));
	f[0][0] = 0;
	for(int i = 0; i < n; ++i){
		for(int j = 0; j <= 5; ++j){
			if(j < 5 && s[i] == t[j]) f[i + 1][j + 1] = min(f[i + 1][j + 1], f[i][j]);
			f[i + 1][j] = min(f[i + 1][j], f[i][j] + min(j, 5 - j));
		}
	}
	
	printf("%d\n", f[n][5]);
	return 0;
}