import java.util.*;
public class Whitespace{
	public static void main(String[] arg){
		Scanner sc = new Scanner(System.in);
		int n,max=0,sum=0,res = 0;
		n = sc.nextInt();
		int[] a = new int[n];
		int[] b = new int[n];
		for(int i = 0;i<n;i++){
			a[i] = sc.nextInt();
			b[i] = sc.nextInt();
			sum = sum + (a[i] % 2);
			if(a[i] + b[i]*4 >max)max = a[i] + b[i] * 4;
		}
		if(sum == n || sum == 0){
			for(int i = 0;i<n;i++){
				res = res + (max - (a[i] + b[i] *4))/2;
			}
			System.out.println(res);
		}else{
			System.out.println(-1);
		}
	}
}