#include<bits/stdc++.h>
//#include"atcoder/all"
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#define all(a) (a).begin(),(a).end()
using namespace std;
//using namespace atcoder;
typedef long long ll;
int main(){
  ll N,i,a,b,ans = 0;cin >> N;
  ll S[N];
  for(i=0;i<N;i++){
    cin >> a >> b;
    S[i] = a+4*b;
  }
  ll B = *max_element(S,S+N);
  for(i=0;i<N;i++){
    if(B%2 != S[i]%2){
      printf("-1\n");return 0;
    }
    ans += (B-S[i])/2;
  }
  cout << ans << "\n";
}