using namespace std;
#include<cstdio>
#include<cstring>
#define NMAX 10005
#define NMAX2 128
char sir[NMAX];
int frecv[NMAX2];
int Sol;
int main()
{
    freopen("litere.in","r",stdin);
    freopen("litere.out","w",stdout);
    int n,i,j,S;
    scanf("%d\n",&n);
    fgets(sir+1,NMAX,stdin);
    frecv[sir[1]]=1;
    for(i=2;i<=n;i++)
    {
        frecv[sir[i]]++;
        S=0;
        for(j=int(sir[i])+1;j<=int('z');j++)
            S+=frecv[j];
        Sol+=S;
    }
    printf("%d\n",Sol);
    return 0;
}