#include<cstdio>
using namespace std;

int n,i,j,vc[27];
char cr;
int perm;

int main(){
    freopen("litere.in","r",stdin);
    freopen("litere.out","w",stdout);

    scanf("%d\n",&n);
    for(i=1;i<=n;i++){
        scanf("%c",&cr);
        cr=cr-'a'+1;
        vc[cr]++;
        for(j=cr+1;j<=26;j++)
            perm+=vc[j];
    }
    printf("%d",perm);
    return 0;
}