def equilibrium_index(arr):
  
  total_sum = sum(arr)
  left_sum=0
  for i, num in enumerate(arr):
    total_sum -= num
    if left_sum == total_sum:
      return i
    left_sum += num
  return -1
def is_not_prime(n):
    import math
    
    result = False
    for i in range(2,int(math.sqrt(n)) + 1):
        if n % i == 0:
            result = True
    return result