#coding: UTF-8

import sys
import re
import itertools
from math import log
from collections import deque

### defs ###


### main ###
N = int(sys.stdin.readline())
X = list(map(int,sys.stdin.readline().split()))
X.sort()
ans=10000000
for i in range(1,N):
	x = X[i]-X[i-1]
	if(x==0):
		continue
	ans = min(ans,x)
if(ans==10000000):
	ans=-1

print(ans)