  We show that the 1d derivative nonlinear Schr\"{o}dinger equation (\ref{equ})
is globally well-posed in $H^s(\mathbb{R})$ for $s\geq 1/2$. We use the
linear-nonlinear decomposition method to take advantage of the local smoothing
effect of the nonlinearity, which enables us to establish a refined version of
the almost conservation law. Note that $H^{1/2}$ is the endpoint that we have
uniform continuous for the solution map and hence our result is sharp.
