#include <bits/stdc++.h>
#define For(x, y, z) for(int x = (y); x <= z; x++)
#define Rep(x, y, z) for(int x = (y); x >= z; x--)
using namespace std;
const int MAXN = 1e5 + 5;
const char t[] = {' ', 's', 'p', 'b', 's', 'u'};
int n; char s[MAXN];
int f[MAXN][6];
int main() {
    scanf("%d%s", &n, s + 1);
    memset(f, 0x3f, sizeof(f)), f[0][0] = 0;
    For(i, 1, n) {
        For(t, 0, 5) f[i][t] = f[i - 1][t];
        if(s[i] == t[1]) f[i][1] = min(f[i][1], f[i - 1][0] - i);
        if(s[i] == t[2]) f[i][2] = min(f[i][2], f[i - 1][1] - i);
        if(s[i] == t[3]) f[i][3] = min(f[i][3], f[i - 1][2]);
        if(s[i] == t[4]) f[i][4] = min(f[i][4], f[i - 1][3] + i);
        if(s[i] == t[5]) f[i][5] = min(f[i][5], f[i - 1][4] + i);
    }
    cout << f[n][5] - 6 << '\n';
    return 0;
}