import java.util.Scanner;

public class Main
{
    private final static Main main = new Main();

    public static void main(String[] args)
    {
        long l = System.currentTimeMillis();
        main.solve();
        //System.out.println("処理時間：" + (System.currentTimeMillis() - l));
    }

    private void solve()
    {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int ans = (sc.nextInt()-sc.nextInt())*-1;
        boolean flag = false;
        for(int i=1; i<n; ++i)
        {
            int x = sc.nextInt();
            int y = sc.nextInt();
            if(ans + x == y)
            {
                flag = true;
            }
            else
            {
                flag = false;
                break;
            }
        }
        System.out.println(flag ? ans : -1);
    }
}