#include<stdio.h>
FILE *f=fopen("litere.in","r"), *g=fopen("litere.out","w");

long int n, i, j, v[40], nr=0, val;
char c;

int main(){

    fscanf(f,"%ld\n",&n);
    for(i=1;i<=n;i++){
        fscanf(f,"%c",&c);
        val=c-'a'+1;
        for(j=val+1;j<=26;j++){
            nr+=v[j];
        }
        v[val]++;
    }
    fprintf(g,"%ld",nr);

return 0;
}