298b.cc

#include <iostream>
#include <algorithm>
#include <cstdlib>
#include <set>
#include <string>
#include <map>
#include <functional>
using namespace std;
int V1, V2, T, D;
int bt(int v, int level, int sum){
	//prune early if v is too high to reduce back to V2
	//note: x/y round up use x+y-1/y
	if(D!=0 && T-1-level < ((v-V2)+(D-1))/D) return -1; 

	if(level==T-1){
		if(v==V2) return sum;
		else return -1;
	}

	int ans;
	for(int i=v+D;i>=v-D;i--){
		if((ans=bt(i, level+1, sum+i))>0){
			return ans;
		}
	}
	return -1;
}

int main(){
	cin>>V1>>V2>>T>>D;
	cout<<bt(V1, 0, V1)<<endl;	
	return 0;
}