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