Simple naive python solution without list slicing:

```
class Solution(object):
def arrayPairSum(self, nums):
"""
:type nums: List[int]
:rtype: int
"""
res = 0
nums = sorted(nums)
# Sum up nums[0], nums[2], nums[4]....
for i in range(0,len(nums)-1,2):
res += nums[i]
return res
```