import java.util.Scanner;

class no185{
	public static void main(String[] args) {
		Scanner stdIn=new Scanner(System.in);
		int n,x,y,i,t=0;
		n=stdIn.nextInt();
		for(i=0;i<n;i++) {
			x=stdIn.nextInt();
			y=stdIn.nextInt();
			if(i>0&&t!=y-x||x>=y) {
				t=-1;
				break;
			}
			t=y-x;
		}
		System.out.print(t);
	}
}