n = int(input())
X = sorted(list(map(int,input().split())))
ret = float('inf')
if len(set(X))==1:
    print(-1)
    exit()
for i in range(1, n):
    if X[i]!=X[i-1]:
        ret = min(ret, abs(X[i] - X[i-1]))
print(ret)