# Python program to print all
# primes smaller than or equal to
# n using Sieve of Eratosthenes
def SieveOfEratosthenes(n):
# Create a boolean array
# "prime[0..n]" and initialize
# all entries it as true.
# A value in prime[i] will
# finally be false if i is
# Not a prime, else true.
prime = [True for i in range(n+1)]
p = 2
while (p * p <= n):
# If prime[p] is not
# changed, then it is a prime
if (prime[p] == True):
# Update all multiples of p
for i in range(p * p, n+1, p):
prime[i] = False
p += 1
# Print all prime numbers
for p in range(2, n+1):
if prime[p]:
print(p)
# Driver code
if __name__ == '__main__':
n = 20
print("Following are the prime numbers smaller"),
print("than or equal to", n)
SieveOfEratosthenes(n)