import java.util.*;

public class Run {
    public static void main (String arg[]) {
        Scanner scan = new Scanner(System.in);
        int N = scan.nextInt();
        int ans = 0;
        int tmpans = 0;
        int X, Y;

        for (int i = 0; i < N; i++) {
            X = scan.nextInt();
            Y = scan.nextInt();
            ans = Y - X;
            if (i == 0) tmpans = ans;
            if (ans <= 0 || ans != tmpans) {
                ans = -1;
                break;
            }
            tmpans = ans;
        }

        System.out.println(ans);
    }
}