#include<iostream>

using namespace std;

#define NMAX 1000

int main(){

	long long N;
	long long a[NMAX],b[NMAX];
	long long spacemax=-1;
	long long ans=0;

	cin>>N;
	for(long long i=0;i<N;i++){
		cin>>a[i]>>b[i];
		if(a[i]+b[i]*4>spacemax) spacemax=a[i]+b[i]*4;
	}

	for(long long i=0;i<N;i++){
		if((spacemax-(a[i]+b[i]*4))%2==1){
			cout<<"-1"<<endl;
			return 0;
		}
		else ans+=(spacemax-(a[i]+b[i]*4))/2;
	}

	cout<<ans<<endl;
}