#include<stdio.h>
int main()
{
	long long n=0,ans=0,a[200005]={0},b[200005]={0},i=0; 
	scanf("%d",&n);
	for(i=1;i<=n;i++)
	{
		scanf("%d",&a[i]);
		if(i=1) b[i]=a[i];
		else b[i]=a[i]+b[i-1];
	}
	
	for(i=1;i<=n-1;i++)
	{
	ans=a[i]*(b[n]-b[i])+ans;
	}
	printf("%d",ans);
	return 0;
}