def repartition(n,a):
    L=tirages(n,a)
    maxL=int(max(L))
    hist(L , bins = 100, range=(0,maxL), density = True,cumulative=True)
    x = [float(i)/100 for i in range(100*maxL)]
    f = [1-exp(-a*t) for t in x]
    plot(x,f)
    show()