#include<bits/stdc++.h>

using namespace std;
using ll=long long;
#define rep2(i, a, n) for(int i = (a); i < (n); i++)
#define rep(i, n) rep2(i,0,n)
template<class T,class U>bool chmax(T &a,const U &b){if(a<b){a=b;return 1;}return 0;}

int main(){
  cin.tie(nullptr);ios_base::sync_with_stdio(false);
  int n;cin>>n;
  int a,b,hoge[n];
  ll shake=0;
  rep(i,n){
    cin>>a>>b;
    if(!a%2){
      cout<<-1<<endl;
      return 0;
    }
    chmax(shake,a+4*b);
    hoge[i]=a+4*b;
  }
  //cout<<shake<<endl;
  ll cnt=0;
  rep(i,n){
    cnt+=shake-hoge[i];
  }
  cout<<(cnt%2?-1:cnt/2)<<endl;
}