#include <stdio.h>

long n,i,j,sol,sigma[27];
char sir[30003];

int main(){
    freopen("litere.in","r",stdin);
    freopen("litere.out","w",stdout);
    
    scanf("%ld\n",&n);
    scanf("%s",sir);
    
    for (i=0;i<n;i++){
        for (j=sir[i]-'a'+1;j<26;j++)sol+=sigma[j];
        sigma[sir[i]-'a']++;
    }
    
    printf("%ld\n",sol);
    
    return 0;
}