void main() {
  import std.stdio, std.string, std.conv, std.algorithm;

  int a, b, c, d;
  rd(a, b, c, d);

  writeln(max(0, min(b, d) - max(a, c)));
}

void rd(T...)(ref T x) {
  import std.stdio : readln;
  import std.string : split;
  import std.conv : to;

  auto l = readln.split;
  assert(l.length == x.length);
  foreach (i, ref e; x)
    e = l[i].to!(typeof(e));
}
