#include <iostream>
using namespace std;
int main(){
	int n;	cin>>n;
	int a[n],b[n],abc[n];
	int max=0;
	for(int i=0;i<n;i++){
		cin>>a[i]>>b[i];
		abc[i]=a[i]+4*b[i];
		max=max>abc[i]?max:abc[i];
	}
	long long int count=0;
	for(int i=0;i<n;i++){
		if((max-abc[i])%2==1){
			cout<<-1<<endl;
			return 0;
		}
		count+=(max-abc[i])/2;
	}
	cout<<count<<endl;
	return 0;
}