#include<stdio.h>

long long n, ct[30], sol, nrord;

void solve()
{
     int i,j;
     char lit;
     freopen("litere.in","r",stdin);
     freopen("litere.out","w",stdout);
     scanf("%d\n",&n);
     for(i=1; i<=n; ++i)
     {
         scanf("%c",&lit);
         nrord= lit- 'a' +1;
         ct[nrord]++;
	 for(j=nrord+1; j<=26; ++j)
	    sol=sol+ ct[j];
     }
     printf("%lld\n", sol);
}

int main()
{
    solve();
    return 0;
}