import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner koko = new Scanner(System.in);
        int n = koko.nextInt();
        int[] x = new int[n];
        int[] y = new int[n];
        for(int i=0; i<n; i++){
            x[i]=koko.nextInt();
            y[i]=koko.nextInt();
        }
        int ans = y[0]-x[0];
        if(ans<=0){
            System.out.println(-1);
        }else{
            boolean wflag = false;
            for(int i=1; i<n; i++){
                int b = y[i]-x[i];
                if(b!=ans){
                    System.out.println(-1);
                    wflag=true;
                    break;
                }
            }
            if(!wflag){
                System.out.println(ans);
            }
        }
    }
}