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# Program to count number of valid pairs from a list of numbers, where pair sum is odd in Python

Suppose we have a list of positive numbers nums, we have to find the number of valid pairs of indices (i, j), where i < j, and nums[i] + nums[j] is an odd number.

So, if the input is like [5, 4, 6], then the output will be 2, as two pairs are [5,4] and [5,6], whose sum are odd.

To solve this, we will follow these steps −

- e := a list by taking only the even numbers in nums
- return (size of nums - size of e) * size of e

Let us see the following implementation to get better understanding −

## Example

class Solution: def solve(self, nums): e=[i for i in nums if i%2==0] return (len(nums)-len(e))*len(e) nums = [5, 4, 6] ob = Solution() print(ob.solve(nums))

## Input

[5, 4, 6]

## Output

2

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