class Solution:
def pivotIndex(self, nums):
"""
:type nums: List[int]
:rtype: int
"""
length = len(nums)
sl = 0
sr = sum(nums) - nums[0]
if sl == sr :
return 0
for i in range(1,length):
sl += nums[i-1]
sr -= nums[i]
if sr == sl:
return i
else :
return -1