Program for Number of stopping station problem

def stopping_station( p, n):

num = 1

dem = 1

s = p

while p != 1:

dem *= p

p-=1

t = n - s + 1

while t != (n-2 * s + 1):

num *= t

t-=1

if (n - s + 1) >= s:

return int(num/dem)

else:

return -1

num = stopping_station(4, 12)

if num != -1:

print(num)

else:

print("Not Possible")