Functional

Hi, Could anyone try to solve this exercise:
among all the crves between points A(-1,0),B(1,0) which lenght is Pi/Sqrt(2), find the one which surface between it and x axis is minimum.
Actually I have solved just a part of this exercise (don't know if it's right)
(1+(y')^2)*Sqrt(1+(y')^2)-y"*(1+(y')^2)+(y')^2=0
Anyway...could anyone explain how to solve this equation?
Thank You.
Regards,
Niko