Smart $\overline{abcdef}$

If $\left(\overline{abc} +\overline{def}\right) ^2=\overline{abcdef}$ , what is value of $a+b+c+d+e-f$ ?

Note : $\overline{abc}, \overline{def}$ are $3$ -digit numbers and $\overline{abcdef}$ is $6$ -digit number and $999$ isn't considered.