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