#include <stdio.h>
int main()
{
  long long N;
  scanf("%lld", &N);
  long long X[N], Y[N];
  for (long long i = 0; i < N; i++)
    scanf("%lld %lld", &X[i], &Y[i]);
  long long i = 0;
  while (Y[i] - X[i] == Y[i + 1] - X[i + 1])
    i++;
  if (i == N - 1)
    printf("%lld", Y[0] - X[0]);
  else
    printf("%d", -1);
  return 0;
}