Summation of ASCII Codes Hash Function

def hash(st):
    st_len = len(st)

    # if there is no text we can return -1 since that's an invalid hash
    if st_len == 0:
        return -1

    # dif has is used to offset the has so input like 'a' would go into bucket 0
    dif_hash = ord('a')

    # if the length is only one we don't need to get the summation of characters 1 to st_len
    if st_len == 1:
        return ord(st) - dif_hash
    else:
        # calculating the summation of the strings ascii values
        #  word[0] + Σ i=1, i < st_len - 'a' ascii value
        summation = 0
        for i in range(1, st_len):
            summation += ord(st[i])
        return ord(st[0]) + summation - dif_hash

while True:
    st = input(">>> ")
    hash_code = (hash(st))
    print(hash_code)