from hashlib import sha384
import time

def strhash (v):
  # v is a string.
  return sha384 (v.encode()).hexdigest()

def proof_of_work (s, dif = 1):
  start_time = time.time()
  i = 0
  while True:
    B = str (i)
    if strhash (s + B)[:dif] == '0' * dif:
      end_time = time.time()
      print(end_time - start_time)
      return B
    i = i + 1