#include<bits/stdc++.h>
using namespace std;
int main(){
	int i,n,a[52],s[28]={0},k=0,j=0,q=0;
	cin>>n;
	for(i=0;i<n;i++){
		cin>>a[i];
	}
	for(i=0;i<n;i++){
		for(j=i+1;j<n;j++){
			s[a[i]+a[j]]++;
		}
	}
	for(i=2;i<=26;i++){
		if(s[i]>0)
		k++;
	}cout<<k<<endl;
	for(i=2;i<=26;i++){
		if(s[i]>0)
		cout<<i<<" ";
	}
	return 0; 
}
/**************************************************************
	Problem: 1334
	User: houshanglin
	Language: C++
	Result: Accepted
	Time:7 ms
	Memory:2072 kb
****************************************************************/