Distance of Haversine

Question

I want to have a Python function to compute the Haversine distance :

def distance_haversine(latitude1, longitude1, latitude2, longitude2):
    a = sin((abs(latitude1-latitude2))/2)**2 + cos(latitude1) * cos(latitude2) * sin(abs(longitude1-longitude2)/2)**2
    c = 2*atan(sqrt(a)/sqrt(1-a))
    return 6371*c

But distance_haversine(0, 100, 50, 80) returns 7428.9064183079645, while the result from this website (sorry, it is in french) is 5875.70 km : https://savvycalculator.com/fr/calculateur-de-distance-haversine/

Thank you for your responses.