#include <stdio.h>
#define F(i,n) for(int i = 0;i < n;i++)
#define A {ans=-1;break;}

int main(void){
	int n,ans; int a[2000];
	scanf("%d",&n);
	F(i,n*2){scanf("%d",&a[i]);}
	ans=a[1]-a[0];
	F(i,n*2){
		if(ans+a[i]!=a[i+1])A
		if(ans<1)A
		i++;
	}
	printf("%d\n",ans);
	return 0;
}