#include <iostream>
#include<algorithm>
#include<map>
using namespace std;
int main()
{
int len, nums[1000] = { 0 }, arm[1000] = { 0 };
map<int, int> mp;
cin >> len;
for (int i = 0; i < len; i++)
{
cin >> nums[i];
if (mp.find(nums[i]) != mp.end())
{
mp[nums[i]]++;
}
else
{
mp[nums[i]] = 1;
}
}
sort(nums, nums + len);
int sum = 0;
for (int i = 0; i < len; i++)
{
if (mp[nums[i]] >= 2)
{
arm[sum] = nums[i];
sum++;
arm[sum] = nums[i];
sum++;
mp[nums[i]] = 0;
}
else if (mp[nums[i]] == 1)
{
arm[sum] = nums[i];
sum++;
}

}
cout << sum << endl;
for (int i = 0; i < sum; i++)
{
cout << arm[i] << " ";
}
}