Implemented in golang, quite easy to be fair 😄

func GetNthFib(n int) int {
    if n == 1 {
        return 0
    }
    if n == 2 {
        return 1
    }
    x, y := 0, 1
    for i := 2; i < n; i++ {
        x, y = y, x+y
    }
    return y
}

Explanation

So we check for the first two value as this two are required to be able to build the rest of the sequence.

Once we know it is not 0 (x) or 1 (y), we know that the correct result comes from adding the x+y and moving the values 1 place forward.

The loop starts at 2 because otherwise the results will be offset by 2 iterations (we already check the first two statically).

For example, if the loop starts at 0 and they ask for nth 3, then our result will be 3 which is incorrect as the third position on Fibonacci's sequence is 1.

You can play around with it by using this Golang Playground link