#include<bits/stdc++.h>
using namespace std;
int a,n,m,x;
int on,off;
int sum=0;
int main(){
	cin>>a>>n>>m>>x;
	if(x<=2) {
		cout<<a<<endl;
		return 0;
	}
	if(x==n){
		cout<<0<<endl;
		return 0;
	}
	on=a+a;
	off=a;
	for(int i=1;i<=x;i++){
		sum+=on;
		sum-=off;
	}
	cout<<sum<<endl;
	return 0;
}