#include <algorithm>
#include <cstdio>
#include <iostream>
#include <map>
#include <cmath>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
#include <stdlib.h>
#include <stdio.h>
#include <bitset>
using namespace std;
#define FOR(I,A,B) for(int I = (A); I < (B); ++I)
typedef long long ll;

//全部奇数or全部偶数ならok

int main(){
  int N;
  cin >> N;
  ll len[N];
  int ok = 0; // 1 -> odd 2 - > even
  FOR(i,0,N) {
    ll a, b;
    cin >> a >> b;
    len[i] = a + 4 * b;
    if(len[i] % 2 == 1 && (ok == 1 || ok == 0)) ok = 1;
    else if(len[i] % 2 == 0 && (ok == 2 || ok == 0)) ok = 2;
    else ok = -1;
  }
  if(ok == -1) {
    cout << -1 << endl;
    return 0;
  }
  sort(len, len + N);
  ll ans = 0;
  FOR(i,0,N-1) {
    ans += len[N-1] - len[i];
  }
  cout << ans / 2 << endl;
  return 0;
}