import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.StringTokenizer;
 
public class Main {
  public static void main(String [] args) {
    System.out.println(solve());
  }
  
  public static long solve() {
    FastReader fr = new FastReader();
    int N = fr.nextInt();
    int[] d = new int[N];
    long sum = 0;
    long result = 0;
    for (int i = 0; i < N; i++) {
      d[i] = fr.nextInt();
      sum += d[i];
    }
    
    for (int i = 0; i < N-1; i++) {
      sum -= d[i];
      result += d[i] * sum;
    }
    
	return result;
  }
}
 
class FastReader {
  BufferedReader br;
  StringTokenizer st;
 
  public FastReader() {
    br = new BufferedReader(new
        InputStreamReader(System.in));
  }
 
  String next() {
    while (st == null || !st.hasMoreElements())
    {
      try
      {
        st = new StringTokenizer(br.readLine());
      }
      catch (IOException e)
      {
        e.printStackTrace();
      }
    }
    return st.nextToken();
  }
 
  int nextInt() {
    return Integer.parseInt(next());
  }
 
  long nextLong() {
    return Long.parseLong(next());
  }
 
  double nextDouble() {
    return Double.parseDouble(next());
  }
 
  String nextLine() {
    String str = "";
    try {
      str = br.readLine();
    }
    catch (IOException e) {
      e.printStackTrace();
    }
    return str;
  }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz