C :

#include<stdio.h>
int main()
{
	int len;
	scanf("%d", &len);
	char c;
	int max = 0;
	for (int i = 0; i <= len; i++) {
		scanf("%c", &c);
		if (c >= max) {
			max = c;
		}
	}
	printf("%d", max - 96);


	return 0;
}

C++ :

#include <stdio.h>

int main()
{
	int n;
	char ch;
	int ans = 0;
	scanf("%d",&n);
	getchar();
	for(int i = 0; i < n; i++)
	{
		scanf("%c",&ch);
		if(ch > ans)
			ans = ch;
	}
	ans = ans - 'a' + 1;
	printf("%d\n",ans);
}