#include<bits/stdc++.h>
using namespace std;

typedef long long int ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<pair<int, int> > vii;
#define rrep(i, m, n) for(int (i)=(m); (i)<(n);  (i)++)
#define erep(i, m, n) for(int (i)=(m); (i)<=(n); (i)++)
#define  rep(i, n)    for(int (i)=0; (i)<(n);  (i)++)
#define rrev(i, m, n) for(int (i)=(n)-1; (i)>=(m); (i)--)
#define erev(i, m, n) for(int (i)=(n); (i)>=(m); (i)--)
#define  rev(i, n)    for(int (i)=(n)-1; (i)>=0; (i)--)
#define vrep(i, c)    for(__typeof((c).begin())i=(c).begin(); i!=(c).end(); i++)
#define  ALL(v)       (v).begin(), (v).end()
#define mp            make_pair
#define pb            push_back
template<class T, class S> inline bool minup(T& m, S x){ return m>(T)x ? (m=(T)x, true) : false; }
template<class T, class S> inline bool maxup(T& m, S x){ return m<(T)x ? (m=(T)x, true) : false; }

const int    INF = 1000000000;
const ll     MOD = 1000000007LL;
const double EPS = 1E-12;

const int MAX_N = 1000;
int N;
int a[MAX_N], b[MAX_N];
int res;
int mx;
int ok;

int main()
{
  cin >> N;
  rep(i, N){
    cin >> a[i] >> b[i];
    if(!ok) ok = (a[i] & 1 ? 1 : -1);
    else if(ok != (a[i] & 1 ? 1 : -1)){
      cout << -1 << endl;
      return 0;
    }
    maxup(mx, a[i] + 4 * b[i]);
  }

  rep(i, N) res += (mx - (a[i] + 4 * b[i])) / 2;

  cout << res << endl;

  return 0;
}