Y=m-x, consider m>0 first. If the straight line is just tangent to the circle, you can see that the distance between the tangent point and the straight line is 0, and the distance between other points on the circle and the straight line is greater than 0. This means that the distance between the tangent point and the straight line is the shortest of all points on the circle.

At this point, when the straight line is translated upwards, it can be found that the original tangent point is still the point with the shortest distance from the straight line. So the distance from the tangent point to the straight line = 1 is the critical condition. If the straight line moves up, the distance from the tangent point to the straight line will be greater than 1, and the distance from other points to the straight line will be greater than 1, which does not meet the requirements in the topic.

Draw a picture by yourself and get m=2√2 under critical conditions. Don't forget that this is m>0, the graph is symmetrical, and M