#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i=0; i<(n); i++)
typedef long long ll;

int main(){
	int n; cin>>n;
	int a,b; int mx=0;
	vector<int> v;
	rep(i,n){
		cin>>a>>b;
		mx=max(mx,(a+b*4));
		v.push_back(a+b*4);
	}
	ll ans=0;
	rep(i,n){
		if(v[i]%2==1){cout<<"-1"<<endl; return 0;}
		ans+=(mx-v[i]);
	}
	cout<<ans/2<<endl;
}