SH likes traveling around the world. When he arrives at a city, he will ask the staff about the number of cities that connected with this city directly. After traveling around a mainland, SH will collate data and judge whether the data is correct.

 A group of data is correct when it can constitute an undirected graph.

There are multiple test cases. The first line of each test case is a positive integer N (1<=N<=10000) standing for the number of cities in a mainland. The second line has N positive integers a1, a2, ...,an. ai stands for the number of cities that connected directly with the ith city. Input will be ended by the END OF FILE.

If a group of data is correct, output "YES" in one line, otherwise, output "NO".

#include<stdio.h> int a[12345]; int main(){    int n,i,j,flag;    while(~scanf("%d",&n))    {        flag=1;        for(i=0;i<n;i++)            scanf("%d",&a[i]);        for(i=0;i<n;i++)        {            for(j=i+1;j<n;j++)            {                if(a[i]&&a[j]){                    a[i]--;                    a[j]--;                }            }            if(a[i]!=0)            {                printf("NO\n");                flag=0;                break;            }        }        if(flag) printf("YES\n");    }    return 0;}  /**************************************************************    Problem: 10    User: Hui    Language: C    Result: Accepted    Time:100 ms    Memory:1012 kb****************************************************************/