// yukicoder: No.145 yukiover
// 2019.8.7 bal4u

#include <stdio.h>

#if 1
#define gc() getchar_unlocked()
#else
#define gc() getchar()
#endif

int in() {   // 非負整数の入力
	int n = 0, c = gc();
	do n = 10 * n + (c & 0xf); while ((c = gc()) >= '0');
	return n;
}

int f[128];
int ans;

void rec(char *s, int a) {
	int c, n; char *p;
	
	if (*s == 0) return;
	n = 0; for (c = a; c < *s; c++) n += f[c];
	p = s-1; while (*++p) if (n > f[*p]) n = f[*p];
	ans += n, p = s; while (*p) f[*p++] -= n;
	
	n = 1000000; p = s; while (*++p) if (n > f[*p]) n = f[*p];
	if (n > f[*s]/2) n = f[*s]/2;
	ans += n, p = s+1; while (*p) f[*p++] -= n;
	rec(s+1, *s+1);
}

int main()
{
	int N;

	N = in();
	while (N--) f[gc()]++;
	rec("ikuy", 'a');
	printf("%d\n", ans + f['z']);
	return 0;
}