#include <bits/stdc++.h>
#define ll long long
#define ff first
#define ss second
#define ld long double
#define pb push_back
#define sws cin.tie(0)->sync_with_stdio(false);
#define endl '\n'

using namespace std;

const int N = 1e5+10;
const ll MOD = 998244353;
// const ll MOD = 1e9+7;
const int INF = 0x3f3f3f3f;
const ll LLINF = 0x3f3f3f3f3f3f3f3f;

ll dp[N][5];
string pat = "spbsu";
ll solve(int i, int k, string &s) {
    if (k == 5) return 0;
    if (i >= (int)s.size()) return LLINF;
    if (dp[i][k] != -1) return dp[i][k];
    ll ans = LLINF;
    if (pat[k] == s[i]) ans = min(ans, solve(i+1, k+1, s));
    ans = min(ans, solve(i+1, k, s) + min(k-0, 5-k));
    return dp[i][k] = ans;
}

int32_t main()
{
    #ifndef LOCAL
    sws;
    #endif

    memset(dp, -1, sizeof(dp));

    int n; cin >> n;
    string s; cin >> s;

    cout << solve(0, 0, s) << endl;


    return 0;
}