def arrayPacking(a):
    #ans = 0
    #for i in range(len(a)):
    #    ans += a[i] << i*8
    #return ans
    return sum([(a[i] << 8*i) for i in range(len(a))])

a = [23, 45, 39]
print(arrayPacking(a))
'''
You are given an array of up to four non-negative integers, each less than 256.

Your task is to pack these integers into one number M in the following way:

    The first element of the array occupies the first 8 bits of M;
    The second element occupies next 8 bits, and so on.

Return the obtained integer M.

Note: the phrase "first bits of M" refers to the least significant bits of M - the right-most bits of an integer. For further clarification see the following example.

Example

For a = [24, 85, 0], the output should be
arrayPacking(a) = 21784.

An array [24, 85, 0] looks like [00011000, 01010101, 00000000] in binary.
After packing these into one number we get 00000000 01010101 00011000 (spaces are placed for convenience), which equals to 21784.
'''