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



int main(){

  int N;
  scanf("%d",&N);
  int i;
  int XY[N][2];
  for (i = 0; i < N; ++i) scanf("%d%d",&XY[i][0],&XY[i][1]);
  if(XY[0][1]-XY[0][0]<=0){printf("-1\n");return 0;}
  int ans;
  ans=XY[0][1]-XY[0][0];
  for (i = 0; i < N; ++i)if(XY[i][1]-XY[i][0]!=ans){printf("-1\n");return 0;}    
  printf("%d\n",ans);
   return 0;
}