#include<stdio.h> 
#include<string.h> 
#include<stdlib.h> 
#include<math.h> 



int main(){
  int N;
  scanf("%d",&N);
  long int ab[N];
  int i,a,b;
  long int max=0;
  for (i = 0; i < N; ++i) {scanf("%d%d",&a,&b); ab[i]=a+b*4;max=fmax(ab[i],max);}
  long int num=0;
  for (i = 0; i < N; ++i) { 
    if((max-ab[i])%2==1){printf("-1\n");return 0;}
    num+=(max-ab[i])/2;    
  }
  printf("%ld\n",num);
    
    
    
  


   return 0;
}