#include <iostream>
#include <algorithm>
#include <cmath>
#include <vector>
#include <complex>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <iomanip>
#include <assert.h>
#include <array>
#include <cstdio>
#include <cstring>
#include <random>
#include <functional>
#include <numeric>
#include <bitset>

using namespace std;

#define REP(i,a,b) for(int i=a;i<(int)b;i++)
#define rep(i,n) REP(i,0,n)
#define all(c) (c).begin(), (c).end()
#define zero(a) memset(a, 0, sizeof a)
#define minus(a) memset(a, -1, sizeof a)
#define minimize(a, x) a = std::min(a, x)
#define maximize(a, x) a = std::max(a, x)

typedef long long ll;
int const inf = 1<<29;

int main() {

  int N; cin >> N;
  int max = 0;
  int as[N], bs[N];
  rep(i, N) {
    int a, b; cin >> a >> b;
    as[i]=a,bs[i]=b;
    maximize(max, a + 4 * b);
  }

  ll ans = 0;
  rep(i, N) {
    if((max-as[i]-4*bs[i]) % 2) { puts("-1"); exit(0); }
    ans += (max-as[i]-4*bs[i]) / 2;
  }

  cout << ans << endl;

  return 0;
}