#include <bits/stdc++.h>
#define ll long long
#define fi first
#define se second
#define endl '\n'
#define MASK(i) (1LL << (i))
#define ull unsigned long long
#define ld long double
#define pb push_back
#define all(x) (x).begin() , (x).end()
#define BIT(x , i) ((x >> (i)) & 1)
#define TASK "task"
#define sz(s) (int) (s).size()
 
using namespace std;
const int mxN = 5e5 + 227;
const int inf = 1e9 + 277;
const int mod = 1e9 + 7;
const ll infll = 1e18 + 7;
const int base = 307;
const int LOG = 20;
 
template <typename T1, typename T2> bool minimize(T1 &a, T2 b) {
    if (a > b) {a = b; return true;} return false;
}
template <typename T1, typename T2> bool maximize(T1 &a, T2 b) {
    if (a < b) {a = b; return true;} return false;
}
 
int n;
string s;
int dp[mxN][10];
char d[5] = {'s', 'p', 'b', 's', 'u'};


void solve()
{  
    cin >> n >> s;
    int res = inf;
    memset(dp, 0x3f, sizeof(dp));
    int temp = dp[0][0];
    dp[0][0] = 0;

    for(int i = 0; i < n; i++) {
        for(int j = 0; j < 5; j++) {
            if(dp[i][j] == temp) continue;
            minimize(dp[i + 1][j], dp[i][j]);
            if(s[i] == d[j]) {
                int cost = 0;
                if(j == 0) cost = -i - 2;
                if(j == 1) cost = -i - 1;
                if(j == 2) cost = 2 * i - i - 2 - i - 1;
                if(j == 3) cost = i;
                if(j == 4) cost = i;
                minimize(dp[i + 1][j + 1], dp[i][j] + cost);
            }
        }
    }
    for(int i = 1; i <= n; i++) {
        minimize(res, dp[i][5]);
    }


    cout << res;
}
 
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    // freopen(TASK".inp" , "r" , stdin);
    // freopen(TASK".out" , "w" , stdout);
 
    int tc = 1;
    //cin >> tc;
    while(tc--) {
        solve();
    }
    return 0;
}