import core.bitop, std.bitmanip;
import core.checkedint;
import std.algorithm, std.functional;
import std.array, std.container;
import std.bigint;
import std.conv;
import std.math, std.numeric;
import std.range, std.range.interfaces;
import std.stdio, std.string;
import std.typecons;
 
void main()
{
    int n;
    readf("%s", &n);
    readln;

    auto arr = readln.chomp.split.map!(to!int).array;
    
    auto s = arr.sum;
    auto ans = 0;
    foreach (e; arr) {
        s -= e;
        ans += e * s;
    }
    
    ans.writeln;
}