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

using uint = unsigned int;
using ll = long long;

#define CIN( LL , A ) LL A; cin >> A 
#define GETLINE( A ) string A; getline( cin , A ) 
#define GETLINE_SEPARATE( A , SEPARATOR ) string A; getline( cin , A , SEPARATOR ) 
#define FOR_LL( VAR , INITIAL , FINAL_PLUS_ONE ) for( ll VAR = INITIAL ; VAR < FINAL_PLUS_ONE ; VAR ++ ) 
#define FOR_ITR( ARRAY , ITR , END ) for( auto ITR = ARRAY .begin() , END = ARRAY .end() ; ITR != END ; ITR ++ ) 
#define REPEAT( HOW_MANY_TIMES ) FOR_LL( VARIABLE_FOR_REPEAT , 0 , HOW_MANY_TIMES ) 
#define RETURN( ANSWER ) cout << ( ANSWER ) << endl; return 0 
#define DOUBLE( PRECISION , ANSWER ) cout << fixed << setprecision( PRECISION ) << ( ANSWER ) << endl; return 0 
#define MIN( A , B ) A < B ? A : B;
#define MAX( A , B ) A < B ? B : A;
template <typename T> inline T Absolute( const T& a ){ return a > 0 ? a : - a; }

constexpr const ll P = 1000000007;

int main(){
  CIN( ll , N );
  CIN( ll , a0 );
  CIN( ll , b0 );
  ll d = a0 % 2;
  ll max = a0 + b0 * 4;
  ll answer = 0;
  FOR_LL( i , 1 , N ){
    CIN( ll , a );
    if( a % 2 != d ){
      RETURN( -1 );
    }
    CIN( ll , b );
    ll num = a + b * 4;
    if( max < num ){
      answer += ( ( num - max ) / 2 ) * i;
      max = num;
    } else {
      answer += ( max - num ) / 2;
    }
  }
  RETURN( answer );
}