#include<stdio.h>



int main(void)
{

	int i,n,a[1001],b[1001],c[1001];

	long long int max=0,sum=0;

	scanf("%d",&n);
	scanf("%d %d",&a[0],&b[0]);
	max=a[0]+4*b[0];
	c[0]=(a[0]+4*b[0])%2;

	for(i=1;i<n;i++){
		scanf("%d %d",&a[i],&b[i]);
		c[i]=(a[i]+4*b[i])%2;
	}

	for(i=1;i<n;i++){
		if(c[i-1]!=c[i]){ printf("-1");return 0;
		}
		if(max<a[i]+4*b[i])	max=a[i]+4*b[i];
	}

	for(i=0;i<n;i++){
		sum=sum+(max-(a[i]+4*b[i]))/2;
	}
	
	printf("%lld",sum);
	return 0;
}