#include <bits/stdc++.h>

inline long nextInt(void) {
  long temp;
  std::cin >> temp;
  return temp;
}

int main() {

  int even = 0;
  int odd  = 0;
  int N;
  int max = -1;
  int ans = 0;
  int temp;
  std::vector<int> v;

  std::cin >> N;
  for(int i = 0; i < N; ++i) {
    temp = nextInt() + 4 * nextInt();
    v.push_back(temp);
    if( temp % 2 == 0 ) ++even;
    else ++odd;
    if( max < temp ) {
      max = temp;
    }
  }

  if( even != 0 && odd != 0 ) {
    std::cout << -1 << std::endl;
    return 0;
  }

  for(int i = 0; i < N; ++i) {
    ans += max - v[i];
  }

  ans /= 2;

  std::cout << ans << std::endl;
  
  return 0;
}