A1011.Fence Painting--Bronze

Several seasons of hot summers and cold winters have taken their toll on
Farmer John's fence, and he decides it is time to repaint it, along with the
help of his favorite cow, Bessie. Unfortunately, while Bessie is actually
remarkably proficient at painting, she is not as good at understanding Farmer
John's instructions.
If we regard the fence as a one-dimensional number line, Farmer John paints
the interval between $x=a$ and $x=b$. For example, if $a=3$ and $b=5$, then
Farmer John paints an interval of length 2. Bessie, misunderstanding Farmer
John's instructions, paints the interval from $x=c$ to $x=d$, which may
possibly overlap with part or all of Farmer John's interval. Please determine
the total length of fence that is now covered with paint.

The first line of the input contains the integers $a$ and $b$, separated by a
space ($a < b$).
The second line contains integers $c$ and $d$, separated by a space ($c < d$).
The values of $a$, $b$, $c$, and $d$ all lie in the range $0 \ldots 100$,
inclusive.

Please output a single line containing the total length of the fence covered
with paint.

Here, 6 total units of fence are covered with paint, from $x=4$ all the way
through $x=10$.