//VRABAC I MACKA
#include <iostream>
using namespace std;
int main()
{
int a, b, c, d;
cin >> a >> b >> c >> d;
int v = b - a;
int m = d - c;
if (v > m)
{
if (a < c)
cout << v - (m + c - a);
else
cout << v - (m - c + a);
}
else
cout << 0;
return 0;
}
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