Directional Derivative

Compute the directional derivative of a scalar function at a point, given the gradient there and a direction. The direction does not need to have length 1.

Implement solve(grad, direction).

Returns. Return a float. A zero direction vector has no direction, so the result is nan.

Examples

Example 1

Input:

solve([3.0, 4.0], [1.0, 0.0])

Output:

3.0

Example 2

Input:

solve([3.0, 4.0], [3.0, 4.0])

Output:

5.0