Google foobar gearing_up_for_destruction

Question

I was doing the google foobar challenge but ran out of time on the following challenge i am trying to see what i did wrong.

Challenge

As

Answer 1

from fractions import Fraction

def answer(a):
  l = len(a)
  if(not a or l == 1): return [-1,-1]
  s = (a[l-1] - a[0]) if (l % 2 == 0) else (-a[l-1]-a[0]); 
  if(l > 2): 
      for i in range(1, l-1): s+= 2 * (-1)**(i+1) * a[i]
  v = Fraction(2*(float(s)/3 if (l%2==0) else float(s))).limit_denominator();
  c = v;
  for i in range(0, l-2):
    d = a[i+1] - a[i]
    n = d - c
    if(c < 1 or n < 1): return [-1,-1]
    else: c = n
  return [v.numerator, v.denominator];