def u(n):
    return 3*n+1
def seuil_explicite(n_0,N):
    n=n_0
    while u(n)<N:
        n=n+1
    return n
def somme_explicite(debut,fin):
    s=0
    for i in range(debut,fin+1):
        s=s+u(i)
    return s