Getting "none type" error while using recursive function

This is the program that I wrote:

1. count_occur() calls first_occur() to get the first occurrence of an element.
2. Then count_occur() calls last_occur() to get the last occurrence
3. Then the index of the first and last occurrence is subtracted to get the number of occurrences.

# calculates the first occurrence of the element
def first_occur(lists, k, low, high): 
    mid = (low+high)//2
    if low > high:
        return -1

    # condition to calculate the first occurrence
    if lists[mid]==k:
        if mid==0 or lists[mid-1]!=lists[mid]:
            return mid
        else:
            return first_occur(lists,k,low,mid-1)
    elif lists[mid]<k:
        first_occur(lists,k,mid+1,high)
    else:
        first_occur(lists,k,low,mid-1)

     
# calculates the last occurrence of the element
def last_occur(lists,k,low,high):
    mid = (low + high)//2
    if low > high:
        return -1

    #condition to check the last occurrence
    if lists[mid] == k:
        if mid == len(lists)-1 or lists[mid+1] != lists[mid]:
            return mid
        else:
            return first_occur(lists,k,mid+1,high)
    elif lists[mid]<k:
        first_occur(lists,k,mid+1,high)
    else:
        first_occur(lists,k,low,mid-1)
        
        
# called via the input
def count_occur(lists,k):
    l = len(lists)
    fo = first_occur(lists,k,0,l)
    print(fo)
    lo = last_occur(lists,k,0,l)
    print(lo)
    
    if fo == -1:
        print('Not found')
    else:
        # checks for the number of occurrences
        print("Found",(fo-lo+1),"times.")
        
        
# getting input via the user of the list.
a = list(map(int, input().split()))
print(a)

k = int(input("Enter the number, the occurences of which to be found."))

count_occur(a, k)

This is the output I am getting. It should have consisted of the number of times the element occurs in the list. But that didn't happen:

12 34 56
[12, 34, 56]
Enter the number, the occurrences of which to be found.12
None
None
Traceback (most recent call last):
  File "/Users/somilsharma/Desktop/DSA/14.py", line 79, in <module>
    count_occur(a,k)
  File "/Users/somilsharma/Desktop/DSA/14.py", line 70, in count_occur
    print("Found",(fo-lo+1),"times.")
                   ~~^~~
TypeError: unsupported operand type(s) for -: 'NoneType' and 'NoneType'