n=int(input())
x=list(map(int,input().split()))
x.sort()
ans = 10**12
for i in range(n-1):
	if x[i+1]-x[i] != 0:
		ans = min(ans, x[i+1]-x[i])
if ans != 10**12:
	print(ans)
else:
	print(-1)