Digits

# Digits drill

function main(n::Int64)
    if (n < 1) || (n > 9); return -1; end
    N = 2*n
    z = Array{Int64}(undef, N - 1)
    for i = 1:n; z[i] = z[N-i] = i; end
    return parse(Int64, join(z, ""))
end