#include<stdio.h>
#define N 10005
int n, suma=0, i, j, w[N], x;
char v[N];
const char in[]= "litere.in";
const char out[]= "litere.out";

int main()
{
	freopen(in,"r",stdin);
	freopen(out,"w",stdout);
	scanf("%d",&n);
	scanf("%s",v);
	 for(i=0;i<n;++i){  
		x=v[i]-'a';  
		++w[x];  
		for(j=x+1;j<='z'-'a';++j)  
		suma+=w[j];  
    }  
			printf("%d\n",suma);
return 0;
}