C++ :

#include<bits/stdc++.h>
using namespace std;
int r,a[105],n,m,sum;
int main()
{
	cin>>r;
	while(r--)
	{
		cin>>n>>m;
		sum=0;
		for(int i=1;i<=m;i++)
		{
			cin>>a[i];	
			sum+=a[i];		
		}
		if(sum%n!=0) {cout<<"impossible"<<endl;continue;}
		sum/=n;
		sort(a+1,a+1+m);
		int bj=0;
		for(int i=m,j=1;i>=j;)
		{
			if(a[i]>sum) {bj=1;break;}
			if(a[i]==sum) i--;
			if(a[i]<sum && a[i]+a[j]>sum) {bj=1;break;}
			if(a[i]<sum && a[i]+a[j]<sum) {bj=1;break;}
			if(a[i]<sum && a[i]+a[j]==sum) i--,j++;
		}
		if(bj==1) cout<<"impossible"<<endl;
		else cout<<"possible"<<endl;
	}
    return 0; 
}