#include <iostream>
#include <vector>
#include <map>
#include <algorithm>
using namespace std;

#define REP(i,s,e) for (i = s; i <= e; i++)
#define rep(i,n) REP (i,0,(int)(n)-1)
#define RREP(i,s,e) for (i = s; i >= e; i--)
#define rrep(i,n) RREP (i,(int)(n)-1,0)
#define INF (int)1e8
#define MOD (int)(1e9+7)

typedef long long ll;

int cnt[256];

int count(string s, char c) {
    int ret = 0;
    if (s.empty()) {
        for (; c >= 'a'; c--)
            ret += cnt[c];
        return ret;
    }
    for (; s[0] < c; c--)
        ret += cnt[c];
    int x = min(cnt[c], count(s.substr(1), c-1));
    ret += x + (cnt[c] - x) / 2;
    return ret;
}

int main(void) {
    int i, n;
    string s;
    cin >> n >> s;

    rep (i,n) cnt[s[i]]++;
    cout << count("yuki",'z') << endl;
    return 0;
}