#include<stdio.h>
#define Nmax 10010
char v[Nmax];
int N,l[100],S,x;
int main()
{
freopen("litere.in","r",stdin);
//freopen("litere.out","w",stdout);
    scanf("%d\n",&N);

    for(int i=1;i<=N;++i)
        {
            scanf("%c",&v[i]);
            x=v[i]-'a'+1;
            ++l[x];
            for(int j=x+1;j<27;++j)
            S+=l[j];
        }
    printf("%d",S);

}