2. Given real numbers $a, b, c, d, e, f$ satisfy the system of equations:
$$
\left\{\begin{array}{l}
2 a+b+c+d+e+f=20, \\
a+2 b+c+d+e+f=40, \\
a+b+2 c+d+e+f=80, \\
a+b+c+2 d+e+f=160, \\
a+b+c+d+2 e+f=320, \\
a+b+c+d+e+2 f=640 .
\end{array}\right.
$$

Then the value of $f-e+d-c+b-a$ is $\qquad$ .
2. 420 .
$$
\begin{array}{l}
\text { (6) - (5) + (4) - (3) + (2)-(1) gives } \\
f-e+d-c+b-a \\
=640-320+160-80+40-20=420 .
\end{array}
$$