#include<cstdio>
using namespace std;
int n,o=1,contor[30],i,j,cnt;
char s[100001];
int main()
{
    freopen("litere.in","r",stdin);
    freopen("litere.out","w",stdout);
    scanf("%d%s",&n,s);
    for(char i='a'; i<='z'; i++)
    {
        contor[i]=o;
        o++;
    }

    for(i=0;i<n;i++)
    {
      for(j=s[i]-'a'+1+1 ;j<=26;j++)cnt+=contor[j];
      contor[s[i]-'a'+1]++;
    }
    printf("%d",cnt);
}