Let a,b,c,d be real numbers such that $ 0 \le a≤b≤c≤d $ and
$c+d =a^2 +b^2 +c^2 + d^2 =1$. Find the maximum value of $a+b$

This was taken from the $2017$ IMC held in India

I attempted to solve this by showing that $ (c+d)^2 =1=c+d$ thus $2cd = a^2 + b^2 $ but Im not sure what to do next