#include <iostream>
#include <string>
#include <vector>
#include <map>
#include <cstring>
#include <functional>
#include <algorithm>
#include <bitset>

int main()
{
	int n;
	std::cin >> n;
	if( n % 2 != 0 ) {
		std::cout << "impossible" << "\n";
		return 0;
	}
		
	std::vector<int> total_skill( n );
	for( int i = 0; i < n; i++ ) {
		int b, p;
		std::cin >> b >>	p;
		total_skill[i] = b + p;
	}
	if( n % 2 != 0 ) {
		total_skill.push_back( 0 );
	}

	std::sort( total_skill.begin(), total_skill.end() );

	int skill_combined = total_skill[0] + total_skill[total_skill.size()-1];

	for( int i = 1; i < total_skill.size()/2; i++ ) {
		if( total_skill[i] + total_skill[total_skill.size() - i - 1] != skill_combined ) {
			std::cout << "impossible" << "\n";
			return 0;
		}
	}
	std::cout << "possible" << "\n";


	return 0;
}