package yukicoder;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Scanner;

public class Yuki178 {
	
	public static void main(String[] args) {
	
			Scanner scan = new Scanner(System.in);
			int n = scan.nextInt();
			int a[] = new int[n];
			int b[] = new int[n];
			boolean a_flg = false;
			boolean b_flg = false;
			
			ArrayList<Integer> array = new ArrayList<Integer>();
			
			for (int i = 0; i < n ; i++ ) {
				a[i] = scan.nextInt();
				b[i] = scan.nextInt();
				
			}
			scan.close();
			
			int ans = 0;
			for (int i = 0; i < n ; i++ ) {
				ans = a[i] + 4 * b[i];
				if(ans != 0) {
					if(ans % 2 == 0 ){
						a_flg = true;
					}else if(ans % 3 == 0 ){
						b_flg = true;
					}
				}
				array.add(ans);
			}
			
			Collections.sort(array);
			int max = Collections.max(array);
			
			ans = 0;
			for(Integer num : array){
				ans += ((max - num) / 2); 
			}
			
			if( a_flg == b_flg == true){
				System.out.println(-1);
			}else {
				System.out.println(ans);
			}
	}

}