#include <stdio.h>
char ch;
int n;
int cate[27];
int main()
{
    freopen ("litere.in","r",stdin);
    freopen ("litere.out","w",stdout);
    scanf("%d",&n);
    scanf("%c",&ch);
    int counter=0;
    for(int i=1;i<=n;i++)
    {
        scanf("%c",&ch);
        for(int i=ch-'a';i<=26;i++) counter+=cate[i];
        cate[ch-'a']=1;
    }
//    std::sort(ch+1,ch+n+1,comp);
    printf("%d\n",counter-1);
}