import java.util.*;
public class Wafu {
	public static void main(String[] arg){
		Scanner sc = new Scanner(System.in);
		int n,temp=0,res=0;
		n = sc.nextInt();
		int[] x = new int[n];
		int[] y = new int[n];
		for(int i = 0;i<n;i++){
			x[i] = sc.nextInt();
			y[i] = sc.nextInt();
		}
		for(int i = 0;i<n;i++){
			if(i ==0){
				temp = y[i] - x[i];
				res = temp;
			}
			if(y[i] - x[i]<= 0 || y[i] - x[i] != temp) {
				res = -1;
				break;
			}
		}
		System.out.println(res);
	}
}