//Normal

#define _GLIBCXX_DEBUG
#define ll long long
#include <bits/stdc++.h>
using namespace std;
using Graph = vector<vector<int>>;

void print(auto a){
  cout << a;
}

void prints(auto a){
  cout << a << " ";
}

void prints(){
  cout << " ";
}

void printl(auto a){
  cout << a << endl;
}

void printl(){
  cout << endl;
}

void fix(int n){
  cout << fixed << setprecision(n);
}

int main(){
  int N; cin >> N;
  vector<vector<int>> X(N, vector<int>(2));

  int maxv = 0;
  for(int i = 0; i < N; i++){
    cin >> X[i][0] >> X[i][1];

    maxv = max(maxv, X[i][0]+X[i][1]*4);
  }

  ll ans = 0;
  for(int i = 0; i < N; i++){
    int diff = maxv - (X[i][0]+X[i][1]*4);

    if(diff%2 != 0){
      printl(-1);

      return 0;
    }

    ans += diff/2;
  }

  printl(ans);

  return 0;
}