import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; public class Main{ public static void main(String[] args) throws IOException { BufferedReader sc = new BufferedReader(new InputStreamReader(System.in)); String n=sc.readLine(); String []s=sc.readLine().split(" "); long sum=0; long sum2=0; for (int i = 0; i < s.length; i++) { sum+=Long.parseLong(s[i])*Long.parseLong(s[i]); sum2+=Long.parseLong(s[i]); } System.out.println((sum2*sum2-sum)/2); } }
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