//#include "stdafx.h"

#include<cstdio>

#include<cctype>

#include<vector>

#include<algorithm>

?#include <queue>

//using namespace std;

??

int n=3;??

int a[1100000]={2021, 11, 21, 10};

int b[1100000]={10, 21, 11, 2021};

?

int main(){

?

int x,i,j;

//*

? scanf("%d",&n);? ?

for( x=0;x<=n;x++)

scanf("%d",&a[x]);?

for( x=0;x<=n;x++)

scanf("%d",&b[x]);??

// */

long long cut=0;

? ?

int tmp=0;

for(i=n;i>=0;i--)

{

tmp=(b[i]-a[i]+tmp+1)/2;

if(tmp>0)cut+=tmp;

else tmp=0;

//printf("%d\n",tmp);

}?

if(tmp)printf("-1\n");

else printf("%lld\n",cut);

??

return 0;

}