Define f(x)={x2+bx+c,x<1x,x≥1 f(x) = \begin{cases} x^2 + bx + c, & x< 1 \\ x, & x \geq 1 \end{cases} f(x)={x2+bx+c,x,x<1x≥1. If f(x) is differentiable at x=1, then b−c is equal to