import sequtils, strutils, algorithm

var
    n = readLine(stdin).parseInt
    x:seq[int] = readLine(stdin).split.map(parseInt)

x = sorted(deduplicate(x), cmp[int])
if x.len < 2:
    echo -1
else:
    echo min(abs(x[0] - x[1]), abs(x[^1] - x[^2]))