#include <iostream>
#include <cstring>
#include <algorithm>
using namespace std;
int main()
{
int n,h[200010]={},s[200010]={};
long long ans=0,temp;
cin>>n;
for(int i=1;i<=n;i++)
{
scanf("%d",&h[i]);
s[i]=s[i-1]+h[i];
}
for(int i=1;i<n;i++)
ans+=(long long)h[i]*(s[n]-s[i]);
cout<<ans<<endl;
return 0;
}