#include <bits/stdc++.h> #define syosu(x) fixed<<setprecision(x) using namespace std; typedef long long ll; typedef unsigned long long ull; typedef pair<int,int> P; typedef pair<double,double> pdd; typedef pair<ll,ll> pll; typedef vector<int> vi; typedef vector<vi> vvi; typedef vector<double> vd; typedef vector<vd> vvd; typedef vector<ll> vl; typedef vector<vl> vvl; typedef vector<string> vs; typedef vector<P> vp; typedef vector<vp> vvp; typedef vector<pll> vpll; typedef pair<int,P> pip; typedef vector<pip> vip; const int inf=1<<30; const ll INF=1ll<<60; const double pi=acos(-1); const double eps=1e-8; const ll mod=1e9+7; const int dx[4]={-1,0,1,0},dy[4]={0,-1,0,1}; int n; vl a; int main(){ cin>>n; a=vl(n); for(int i=0;i<n;i++){ int A,B; cin>>A>>B; a[i]=A+4*B; } sort(a.rbegin(),a.rend()); ll res=0; for(int i=1;i<n;i++){ int t=a[0]-a[i]; if(t%2==1) res=-INF; else res+=t/2; } cout<<(res<0?-1:res)<<endl; }