#include <bits/stdc++.h>
#define REP(i,n,N) for(int i=(n);i<(int) N;i++)
#define p(s) cout<<(s)<<endl
typedef long long ll;
using namespace std;

int main(){
	int N;
	cin>>N;
	ll a[1010],b[1010];
	ll mx=0;
	REP(i,0,N){
		cin>>a[i]>>b[i];
		mx=max(mx,a[i]+4*b[i]);
	}
	ll ans=0;
	REP(i,0,N){
		int now=a[i]+4*b[i];
		if((mx-now)%2==0) ans+=(mx-now)/2;
		else{
			ans=-1;
			break;
		}
	}
	p(ans);
	return 0;
}