#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; n--;
	int a,b; cin>>a>>b;
	vector<int> v;
	v.push_back(a+4*b);
	bool od=false; bool ev=false;
	if(v[0]%2==0) ev=true;
	else od=true;
	int mx=v[0];
	rep(i,n){
		cin>>a>>b;
		v.push_back(a+4*b);
		if(v[i+1]%2==0&&od==true){cout<<"-1"<<endl; return 0;}
		if(v[i+1]%2==1&&ev==true){cout<<"-1"<<endl; return 0;}
		mx=max(mx,v[i+1]);
	}
	n++;
	ll ans=0;
	for(auto x:v) ans+=mx-x;
	cout<<ans/2<<endl;
}