#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
const int INF = 1e9;
const ll LINF = 1e18;
template<class S,class T> ostream& operator << (ostream& out,const pair<S,T>& o){ out << "(" << o.first << "," << o.second << ")"; return out; }
template<class T> ostream& operator << (ostream& out,const vector<T> V){ for(int i = 0; i < V.size(); i++){ out << V[i]; if(i!=V.size()-1) out << " ";} return out; }
template<class T> ostream& operator << (ostream& out,const vector<vector<T> > Mat){ for(int i = 0; i < Mat.size(); i++) { if(i != 0) out << endl; out << Mat[i];} return out; }
template<class S,class T> ostream& operator << (ostream& out,const map<S,T> mp){ out << "{ "; for(auto it = mp.begin(); it != mp.end(); it++){ out << it->first << ":" << it->second; if(mp.size()-1 != distance(mp.begin(),it)) out << ", "; } out << " }"; return out; }

/*
 <url:https://yukicoder.me/problems/no/145>
 問題文============================================================
 =================================================================
 解説=============================================================
 ================================================================
 */

ll alpha[26];
ll solve(){
    ll res = 0;
    ll N; string S; cin >> N >> S;
    for(char c:S) alpha[c-'a']++;
    string yuki = "zyuki";
    for(int n = 4; n >= 1; n--){
        while(true){
            bool ok = true;
            for(int m = 1; m < n;m++){
                if(alpha[yuki[m]-'a']) continue;
                ok = false; break;
            }
            if(!ok) break;
            
            for(int m = 1; m < n;m++) alpha[yuki[m]-'a']--;
            
            bool update = false;
            for(int x = yuki[n]+1; x <= yuki[n-1]; x++){
                if(alpha[x-'a']){
                    res++;
                    alpha[x-'a']--;
                    update = true;
                    break;
                }
            }
            if(update) continue;
            for(int m = 1; m < n;m++) alpha[yuki[m]-'a']++;
            break;
        }
    }
    return res;
}
int main(void) {
    cin.tie(0); ios_base::sync_with_stdio(false);
    cout << solve() << endl;
    return 0;
}