import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintStream;
import java.io.PrintWriter;
import java.lang.reflect.Array;
import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.Objects;
import java.util.Queue;
import java.util.Random;
import java.util.function.IntBinaryOperator;
import java.util.function.IntFunction;
import java.util.function.IntToDoubleFunction;
import java.util.function.IntToLongFunction;
import java.util.function.IntUnaryOperator;
import java.util.function.LongBinaryOperator;
import java.util.function.Predicate;
import java.util.function.ToIntFunction;
import java.util.random.RandomGenerator;
import java.util.stream.IntStream;
import java.util.stream.Stream;

public class Main {
    static MyPrintWriter pw = MyPrintWriter.getInstance();

    static FastScanner sc = FastScanner.getInstance();

    public static void main(String[] args) throws IOException {
        Thread.setDefaultUncaughtExceptionHandler((t, e) -> System.exit(1));
        new Main().run();
        pw.flush();
    }

    void run() {
        int N = sc.nextInt();
        int[] A = sc.nextInts(N);
        int sumA = ((int) (ArrayUtils.sum(A)));
        long ans = ((1L * sumA) * (sumA - 1)) / 2;
        for (int i = 0; i < A.length; i++) {
            long a = A[i];
            ans -= (a * (a - 1)) / 2;
        }
        pw.println(ans);
    }
}

class ArrayUtils {
    public static long sum(int[] a) {
        long ret = 0;
        for (int val : a) {
            ret += val;
        }
        return ret;
    }
}

class FastScanner {
    private static FastScanner instance = null;

    private final InputStream in = System.in;

    private final byte[] buffer = new byte[1 << 16];

    private int ptr = 0;

    private int buflen = 0;

    private FastScanner() {
    }

    public static FastScanner getInstance() {
        if (instance == null) {
            instance = new FastScanner();
        }
        return instance;
    }

    private boolean hasNextByte() {
        if (ptr < buflen) {
            return true;
        }
        ptr = 0;
        try {
            buflen = in.read(buffer);
        } catch (IOException e) {
            e.printStackTrace();
        }
        return buflen > 0;
    }

    private int readByte() {
        if (hasNextByte()) {
            return buffer[ptr++];
        } else {
            return -1;
        }
    }

    private boolean isPrintableChar(int c) {
        return (33 <= c) && (c <= 126);
    }

    public boolean hasNext() {
        while (hasNextByte() && (!isPrintableChar(buffer[ptr]))) {
            ptr++;
        } 
        return hasNextByte();
    }

    public long nextLong() {
        if (!hasNext()) {
            throw new NoSuchElementException();
        }
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        while ((b >= '0') && (b <= '9')) {
            // n = n * 10 + (b - '0');
            n = ((n << 1) + (n << 3)) + (b - '0');
            b = readByte();
        } 
        return minus ? -n : n;
    }

    public int nextInt() {
        return ((int) (nextLong()));
    }

    public int[] nextInts(int n) {
        int[] a = new int[n];
        for (int i = 0; i < n; ++i) {
            a[i] = nextInt();
        }
        return a;
    }
}

class MyPrintWriter extends PrintWriter {
    private static MyPrintWriter instance = null;

    private MyPrintWriter() {
        super(System.out);
    }

    public static MyPrintWriter getInstance() {
        if (instance == null) {
            instance = new MyPrintWriter();
        }
        return instance;
    }
}


// --- Original Code ---
// 
// 
// import java.io.IOException;
// import java.util.ArrayList;
// import java.util.Arrays;
// 
// import library.tools.FastScanner;
// import library.tools.MergeFiles;
// import library.tools.MyPrintWriter;
// import library.util.ArrayUtils;
// import library.util.Fp;
// import library.util.collections.LongArrayList;
// import library.util.fold.StaticPointAddRangeSum1D;
// import library.util.graph.ImplicitDigraph;
// 
// public class Main {
// 	static MyPrintWriter pw = MyPrintWriter.getInstance();
// 	static FastScanner sc = FastScanner.getInstance();
// 
// 	public static void main(String[] args) throws IOException {
// 		new Main().run();
// 		pw.flush();
// 		MergeFiles.export();
// 	}
// 	
// 
// 	void run() {
// 		int N=sc.nextInt();
// 		int[]A=sc.nextInts(N);
// 		int sumA=(int)ArrayUtils.sum(A);
// 		long ans=1L*sumA*(sumA-1)/2;
// 		for (int i = 0; i < A.length; i++) {
// 			long a=A[i];
// 			ans-=a*(a-1)/2;
// 		}
// 		pw.println(ans);
// 	}
//     
// 	void tr(Object... objects) {
// 		System.out.println(Arrays.deepToString(objects));
// 	}
// }
//