#include <iostream>
using namespace std;
int main()
{
	int a,b,c,d;
	cin>>a>>b>>c>>d;
	if(c<d)
		cout<<(d-c)*2*b+(d-c-1)*a;
	else
		if(c==d)
			cout<<a;
		else
			cout<< (c-d)*2*b+(c-d+1)*a;
	return 0;
}