/**************************************************************

    Problem: 1211

    User: walfy

    Language: Java

    Result: Accepted

    Time:560 ms

    Memory:17892 kb

****************************************************************/

import java.math.BigInteger;

import java.util.*;

public class Main {

    public static void main(String[] args) {

        Scanner cin = new Scanner(System.in);

        int n = cin.nextInt();

        int[] a = new int [];

        boolean ok = true;

        int sum=;

        for(int i=;i<n;i++)

        {

            a[i]=cin.nextInt();

            if(a[i] == )ok=false;

            sum += a[i];

        }

        if(n==)

        {

            if(a[]==)System.out.println();

            else System.out.println();

            return ;

        }

        if(ok == false||sum != *n-)

        {

            System.out.println();

            return ;

        }

        BigInteger [] b = new BigInteger [];

        b[] = BigInteger.ONE;

        for(int i=;i<;i++)

        {

            b[i]=b[i-].multiply(BigInteger.valueOf(i));

        }

        BigInteger ans = b[n-];

        for(int i=;i<n;i++)

        {

            ans=ans.divide(b[a[i]-]);

        }

        System.out.println(ans);

    }

}