#include<stdio.h>
int main(void){
  int n;
  int i;
  scanf("%d",&n);
  int x[n],y[n];
  int difference[n];

  for(i=0;i<n;i++){
    scanf("%d %d",&x[i],&y[i]);
  }
  for(i=0;i<n;i++){
    difference[i]=y[i]-x[i];
  }
  for(i=0;i<n-1;i++){
    if(difference[i]!=difference[i+1]){
      printf("-1\n");
      return;
    }
  }
  printf("%d\n",difference[0]);
  return 0;
}