#include <iostream>
#include <vector>
using namespace std;
typedef long long LL;
int main(int argc, char* argv[])
{
	LL i,N;
	cin>>N;
	vector<LL> C;
	LL A,B;
	LL Max=0;
	for (i=0;i<N;i++){
		cin>>A>>B;
		LL c=A+4*B;
		C.push_back(c);
		if (Max<c){
			Max=c;
		}
	}
	LL num=0;
	for (i=0;i<N;i++){
		LL num1=Max-C[i];
		if (num1%2==0){
			num+=num1/2;
		}else{
			cout<<-1<<endl;
			return 0;
		}
	}
	cout<<num<<endl;
	return 0;
}