from numpy import diff


def main():
    N = int(input())
    X = list(map(int, input().split()))
    X.sort()

    X_diff = diff(X)
    X_diff_positive = list(filter(lambda num: num > 0, X_diff))
    if not X_diff_positive:
        print(-1)
    else:
        print(min(X_diff_positive))


if __name__ == "__main__":
    main()