#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;
	scanf("%d", &N);
	char *S = new char[N+1];
	scanf("%s", S);
	const int A = 26;
	vector<int> cnt(A, 0);
	const char *yuki = "yuki`";
	const int K = 5;
	rep(i, N) ++ cnt[S[i] - 'a'];
	int ans = 0;
	bool changed = true;
	while(changed) {
		changed = false;
		int prefix = N * 2;
		rep(i, K) {
			int c = yuki[i] - 'a';
			int t = 0;
			reu(a, c + 1, A)
				t += cnt[a];
			//prefixを1減らしてtをi増やせる
			amin(t, prefix);
			if(t != 0) changed = true;
			ans += t;
			prefix -= t;
			reu(a, c + 1, A) {
				int u = min(t, cnt[a]);
				t -= u;
				cnt[a] -= u;
			}
			if(i < K-1) {
				amin(prefix, cnt[c]);
				cnt[c] -= prefix;
			}
		}
		rep(j, K-1) cnt[yuki[j] - 'a'] += prefix;
	}
	printf("%d\n", ans);
	return 0;
}