#include<fstream.h>

int main()
{
 ifstream fin("litere.in");
 ofstream fout("litere.out");
 int n,i,j;
 char s[10002];
 long contor=0;
 fin>>n;
 fin.get();
 fin.get(s,n+1);
 fin.close();
 for(i=0;i<=n-1;i++)
	for(j=0;j<i;j++)
		if((s[i]-'a')<(s[j]-'a'))
			contor++;
 fout<<contor;
 fout.close();
return 0;
}