Given a sequence of numbers, find the largest pair sum in the sequence.
For example
```
[10, 14, 2, 23, 19] -->  42 (= 23 + 19)
[99, 2, 2, 23, 19]  --> 122 (= 99 + 23)
```
Input sequence contains minimum two elements and every element is an integer.
from heapq import nlargest

def largest_pair_sum(a):
    return sum(nlargest(2, a))
from heapq import nlargest

def largest_pair_sum(a):
    return sum(nlargest(2, a))