def longueur(xA,yA,xB,yB):

    return ( (xB-xA)**2 + (yB-yA)**2 )**0.5



def longTotale(points):

    somme = ...

    prem = ...

    for key,value in points.items():

        if prem:

            previous = ...

            prem = ...

        else:

            somme += ...

            previous = ...



    return somme



points = {

    'A' : (-2,4),

    'B' : (1,-2),

    'C' : (3,7),

    'D' : (5,-3)

}



print( longTotale(points) )