Min value of $F(x) = x - \sqrt{1-x^2} $

But using derivatives $F(x) = x - \sqrt{1-x^2} $

i stucked at $2\sqrt{1-x^2} + 2x = 0$

$\sqrt{1-x^2} > 0 $

I get $-1<x<1$

Please help me?
 is there another way to solve it btw?