Project Euler 8: Getting Incorrect Answer From Code

I'm currently trying to do some of the Project Euler questions. I'm currently on question 8 and I'm using python to solve it.

I made some code and got an answer, which turns out to be incorrect. I'm not sure what the problem is with my code, so I'd be grateful is someone could tell me what the issue is. I'm getting an answer of 56435097600, which is the 13 numbers multiplied together on the 30th column with an offset of 6, so it starts 7894... Here's my code, it's not very efficient or neat, but I just want to know what the issue is, not change my code to make it neater/more effective.

rows = ['73167176531330624919225119674426574742355349194934','96983520312774506326239578318016984801869478851843','85861560789112949495459501737958331952853208805511',
        '12540698747158523863050715693290963295227443043557','66896648950445244523161731856403098711121722383113','62229893423380308135336276614282806444486645238749',
        '30358907296290491560440772390713810515859307960866','70172427121883998797908792274921901699720888093776','65727333001053367881220235421809751254540594752243',
        '52584907711670556013604839586446706324415722155397','53697817977846174064955149290862569321978468622482','83972241375657056057490261407972968652414535100474',
        '82166370484403199890008895243450658541227588666881','16427171479924442928230863465674813919123162824586','17866458359124566529476545682848912883142607690042',
        '24219022671055626321111109370544217506941658960408','07198403850962455444362981230987879927244284909188','84580156166097919133875499200524063689912560717606',
        '05886116467109405077541002256983155200055935729725','71636269561882670428252483600823257530420752963450',]

columns  = []

def createcolumns():

    global rows,columns

    for x in range(0,50):
        tempvalue = ''
        for j in range(0,20):
            tempvalue = tempvalue + rows[j][x]
        columns.append(tempvalue)

def multiply(string):
    tempvalue = 1
    for letters in string:
        tempvalue *= int(letters)
    return tempvalue


def highestnumber():

    global rows,columns

    highest = 0

    # 37 ways per row * 20 + 7 ways per column * 50

    for x in range(0,20):
        for g in range(0,37):
            tempvalue = ''
            for j in range(0,13):
                tempvalue = tempvalue + rows[x][j+g]
            tempvalue = multiply(tempvalue)
            if(tempvalue > highest):
                highest = tempvalue

    for x in range(0,50):
        for g in range(0,7):
            tempvalue = ''
            for j in range(0,13):
                tempvalue = tempvalue + columns[x][j+g] 
            tempvalue = multiply(tempvalue)
            if(tempvalue > highest):
                highest = tempvalue

    return highest

createcolumns()
print(highestnumber())

Output:

>> 56435097600