Fix Python – Getting distance between two points based on latitude/longitude

I tried implementing this formula: http://andrew.hedges.name/experiments/haversine/
The aplet does good for the two points I am testing:

Yet my code is not working.
from math import sin, cos, sqrt, atan2

R = 6373.0

lat1 = 52.2296756
lon1 = 21.0122287
lat2 = 52.406374
lon2 = 16.9251681

dlon = lon2 – lon1
dlat = lat2 – lat1
a = (sin(dlat/2))**2 ….