#include <algorithm>
#include <cstdio>
#include <cstdlib>
#include <cctype>
#include <cmath>
#include <iostream>
#include <queue>
#include <list>
#include <map>
#include <numeric>
#include <set>
#include <sstream>
#include <string>
#include <vector>
using namespace std;
#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define FOR(it,c) for(__typeof((c).begin()) it=(c).begin(); it!=(c).end(); ++it)
#define ALLOF(c) (c).begin(), (c).end()
typedef long long ll;


int A[1005], B[1005];
int main(){
  int N;
  cin >> N;
  int mx = 0;
  rep(i,N){
    cin >> A[i] >> B[i];
    mx = max(mx, A[i]+4*B[i]);
  }

  int ret = 0;
  rep(i,N){
    if((mx-(A[i]+4*B[i]))%2==1){
      cout << -1 << endl;
      return 0;
    }
    ret += (mx-(A[i]+4*B[i]))/2;
  }

  cout << ret << endl;
  return 0;
}