#include<stdio.h>
int j,N,f[100000],sum;
char litera;
int main()
{
    freopen("litere.in","r",stdin);
    freopen("litere.out","w",stdout);
    scanf("%d\n",&N);
    while(N--)
    {
              scanf("%c",&litera);
              for(j=litera;j>=litera-'a'+1;j--)
              if(f[j]) sum+=f[j];
              f[litera]++;
    }
    printf("%d",sum);
    return 0;
}