#include <string>
#include <vector>
#include <algorithm>
#include <numeric>
#include <set>
#include <map>
#include <queue>
#include <iostream>
#include <sstream>
#include <cstdio>
#include <cmath>
#include <ctime>
#include <cstring>
#include <cctype>
#include <cassert>
#include <limits>
#include <functional>
#define rep(i,n) for(int (i)=0;(i)<(int)(n);++(i))
#define rer(i,l,u) for(int (i)=(int)(l);(i)<=(int)(u);++(i))
#define reu(i,l,u) for(int (i)=(int)(l);(i)<(int)(u);++(i))
#if defined(_MSC_VER) || __cplusplus > 199711L
#define aut(r,v) auto r = (v)
#else
#define aut(r,v) __typeof(v) r = (v)
#endif
#define each(it,o) for(aut(it, (o).begin()); it != (o).end(); ++ it)
#define all(o) (o).begin(), (o).end()
#define pb(x) push_back(x)
#define mp(x,y) make_pair((x),(y))
#define mset(m,v) memset(m,v,sizeof(m))
#define INF 0x3f3f3f3f
#define INFL 0x3f3f3f3f3f3f3f3fLL
using namespace std;
typedef vector<int> vi; typedef pair<int,int> pii; typedef vector<pair<int,int> > vpii; typedef long long ll;
template<typename T, typename U> inline void amin(T &x, U y) { if(y < x) x = y; }
template<typename T, typename U> inline void amax(T &x, U y) { if(x < y) x = y; }

int main() {
	int N;
	while(~scanf("%d", &N)) {
		char *S = new char[N + 1];
		scanf("%s", S);
		int cnt[26] = {};
		rep(i, N) {
			char c = S[i];
			++ cnt[c - 'a'];
		}
		const char *yuki = "yuki`";
		int ans = 0;
		int prevnum = INF;
		rep(k, 5) {
			int c = yuki[k] - 'a';
			int t = 0;
			reu(i, c + 1, 26) {
				int u = cnt[i];
				rep(l, k)
					if(yuki[l] - 'a' == i)
						u -= prevnum;
				t += u;
			}
			//min(t + p * k, prevnum - p)
			//p <= (prevnum - t) / (k+1)
			int p = max(0, prevnum - t) / (k + 1);
			int u = min(t + p * k, prevnum - p);
			ans += u;
			rep(l, k)
				cnt[yuki[l] - 'a'] -= u;
			int remu = u;
			reu(i, c + 1, 26) {
				int p = min(remu, cnt[i]);
				cnt[i] -= p;
				remu -= p;
			}
			if(k < 4)
				prevnum = min(prevnum - u, cnt[c]);
		}
		printf("%d\n", ans);
	}
	return 0;
}