#include<stdio.h>

int main(void){
  int i;
  int N, X, Y;
  int A[1000];
  int flag = 0;
  scanf("%d", &N);
  for(i = 0; i < N; i++){
    scanf("%d %d", &X, &Y);
    A[i] = Y - X;
  }
  for(i = 1; i < N; i++){
    if(A[0] != A[i]){
      flag = 1;
      break;
    }
  }
  if(flag == 1) printf("-1\n");
  else if(A[0] < 0) printf("-1\n");
  else printf("%d\n", A[0]);
  return 0;
}