输出不小于某个数的最大质数

import math

def main(n):
    ret = []
    flag = [1]*(n+1)
    for i in range(2, n+1):
        if flag[i]:
            ret.append(i)

        for j in range(2, int(round(math.sqrt(n+1)))):
            if i*j<=n:
                flag[i*j] = 0

    #print(ret)
    return ret[-1]

if __name__ == '__main__':
    import sys
    print main(int(sys.argv[1]))