#include<cstdio>
#include<algorithm>
#include<cstring>
#define Nmax 10005
using namespace std;
int n,i,j,nr;
char s[Nmax];
int w[28];
int main()
{
    freopen("litere.in","r",stdin);
    freopen("litere.out","w",stdout);
    scanf("%d\n",&n);
    gets(s+1);
    for (i=1;i<=n;i++)
    {
        for (j=s[i]-'a'+1;j<=26;j++)
        nr+=w[j];
        ++w[s[i]-'a'];
    }
    printf("%d",nr);
    return 0;
}