An even permutation is a permutation on a finite set satisfying the equivalent conditions: It can also be expressed as a product of an even number of transpositions. For an even permutation, the number of cycles of even length in its cycle decomposition is even. The below given is the even permutation formula for calculating the permutation on your own. As per the formula, just calculate the factorial for your n and divide it by 2 get the permutation.
It is possible to say the whether the number is even or odd only after swapping it. The above even permutation formula is a simple formula for which requires only factorial and division operations to perform calculations.