#!/usr/bin/env python3


class Question:
    def __init__(self,i=1, l=0, n=''):
        self.name=n
        self.level=l
        self.number=i
    def __enter__(self):
        print('\n' + (self.level*2)*' ' + f"-> {self.number}. : {self.name} -- Début")
        return self
    def __exit__(self, exc_type, exc_value, exc_traceback):
        print((self.level*2)*' ' + f"<- {self.number}. : {self.name} -- Fin\n")

with Question(2):
    L = [True]*31
    def modifier(L, p):
        if p in (0,1):
            L[p] = False
        else:
            for i in range(2*p,len(L), p):
                L[i] = False
    def premiers(n):
        L = [True]*(n+1)
        for i in range(n):
            if L[i]:
                modifier(L, i)
        return [i for i, premier in enumerate(L) if premier]
    print(premiers(823417)[-1])
    print("Donc 823417 n'est pas premier.")