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

typedef long long ll;

signed main(){
  int N; cin >> N;
  vector< int > A( N ), B( N );
  for( int i = 0; i < N; ++i )
    cin >> A[ i ] >> B[ i ];
  vector< int > val( N );
  for( int i = 0; i < N; ++i )
    val[ i ] = A[ i ] + 4 * B[ i ];
  int maxv = *max_element( val.begin(), val.end() );
  int ans = 0;
  for( int i = 0; i < N; ++i ){
    int d = maxv - val[ i ];
    if( d & 1 )
      cout << -1 << endl,
      exit( 0 );
    ans += d / 2;
  }
  cout << ans << endl;
  return 0;
}