#include <iostream>
#include <string>
#include <cmath>
#include <set>
#include <map>
#include <algorithm>
#include <cstring>
#include <vector>
using namespace std;
typedef long long ll;
const int N =1e3 +5;
const int M = 1e9 +7;
const int inf = 0x3fffffff;
void solve(){
    int h,k,v,s;
    cin>>h>>k>>v>>s;
    ll ss=0;
    while(h>0){
    	v+=s;
        v-=max(1,(v/10));
        if(v>=k){
            h++;
        } 
		else if(v>0&&v<k){
            h--;
            if(h==0){
                v=0;
            }
        }
        if(v<=0){
            h=0;
            v=0;
        }
        ss+=v;
        if(s>0){
            s--;
        }
	}
	cout<<ss<<"\n";
}
int main(){
    ios::sync_with_stdio(0);cin.tie(0);
    //int t;cin>>t;while(t--)
    solve();
    return 0;
}