#include<stdio.h>
using namespace std;

int v[30];

int main()
{
    freopen("litere.in","r",stdin);
    freopen("litere.out","w",stdout);
    int n,i,j,s=0;
    char c;
    scanf("%d\n",&n);
    for(i=1;i<=n;i++)
    {
        scanf("%c",&c);
        ++v[c-96];
        for(j=c+1-96;j<='z'-96;++j)
                s+=v[j];
    }
    printf("%d\n",s);
    return 0;
}