import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int [] X = new int [N];
        int [] Y = new int [N];
        X[0] = sc.nextInt();
        Y[0] = sc.nextInt();
        int d = Y[0] - X[0];
        int r = d;
        if(d>=1){
            for(int i=1; i<N; i++){
                X[i] = sc.nextInt();
                Y[i] = sc.nextInt();
                int temp = Y[i] - X[i];
                if(d!=temp){
                    r=-1;
                    break;
                }
            }
        }else{
            r=-1;
        }
        System.out.println(r);
    }
}