ERROR 000989 Issue with Python Script and Field Calculating distance between two long lat points

very new to scripting and I have been struggling with my script for a while. I'm not sure why it doesn't work in field calculator. I am trying to calculate a list of distance between two long lat points. The following is the code that I have implemented in the Pre-Logic Script Code for FieldName1. I am currently using ArcGIS 10.1

def Distance(Latitude, Latitude2, Longitude, Longitude2)
Longitude, Latitude, Longitude2, Latitude2 = map(math.radians, [Longitude, Latitude, Longitude2, Latitude2])
dlong = Longitude2-Longitude
dlat = Latitude2 - Latitude
a = math.sin(dlat/2)**2 + math.cos(Latitude * math.cos(Latitude2) * math.sin(dlong/2)**2
c = 2 * math.asin(math.sqrt(a))
Distance = 6371 * C * 1000

The following are sample points for reference: Latitude :48.5761 Longitude : -124.31111 Latitude 2: 48.57611 Longitude 2: -124.31111

I kept getting error 000989: Python syntax error: Parsing error SyntaxError:invalid syntax(line1). It also adds that the parameter are not valid.

According to this tutorial the map function returns a list:

>>> items = [1, 2, 3, 4, 5]
>>>
>>> def sqr(x): return x ** 2

>>> list(map(sqr, items))
[1, 4, 9, 16, 25]
>>>

So your code should be something like this:

def Distance(Latitude, Latitude2, Longitude, Longitude2)
pList = map(math.radians, [Longitude, Latitude, Longitude2, Latitude2])
Longitude  = pList
Latitude   = pList
Longitude2 = pList
Latitude2  = pList
dlong = Longitude2-Longitude
dlat = Latitude2 - Latitude
a = math.sin(dlat/2)**2 + math.cos(Latitude * math.cos(Latitude2) * math.sin(dlong/2)**2
c = 2 * math.asin(math.sqrt(a))
Distance = 6371 * C * 1000

Which would be the same as writing:

def Distance(Latitude, Latitude2, Longitude, Longitude2)