#include <iostream>
using namespace std;
int main() {
int a,b,k,x;
cin>>a>>b>>k>>x;
int max=k+x;
int min=k-x;
if(max<a)
cout<<"IMPOSSIBLE";
else if(max>=b && a<=min)
cout<<b-min+1;
else if(max<=a && max<=b)
cout<<max-a+1;
else if(min <=a && max>=b)
cout<<b-a+1;
else
cout<<max-min+1;
return 0;
}
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