#!/usr/bin/env python2

import math


def BLEU(N, w, v):
  sum = 0
  for i in range(N):
    j = i+1
    if v[i] == 0: continue
    sum += (w[i]*math.exp(math.log(v[i])))/(2**(N-j+1))
    print v[i], math.log(v[i]), w[i] * math.log(v[i]), w[i]
  return sum

N = 4
w = [1.0/N for i in range(N)]
v = [0.1, 0.1, 0.1, 0.1]
print BLEU(N, w, v)