#include <bits/stdc++.h>
using namespace std;

#define SZ(a) (int)a.size()
#define ALL(a) a.begin(), a.end()
#define FOR(i, a, b) for (int i = (a); i<(b); ++i)
#define RFOR(i, b, a) for (int i = (b)-1; i>=(a); --i)
#define MP make_pair
#define PB push_back
#define F first
#define S second

typedef long long LL;
typedef pair<int, int> PII;
typedef vector<int> VI;

int main()
{
	ios::sync_with_stdio(false);
	cin.tie(0);
	LL h, k, v, s;
	cin >> h >> k >> v >> s;
	LL x = 0;
	while (h > 0) {
		v += s;
		v -= max(1LL, v / 10);
		if (v >= k) {
			h++;
		}
		if (0 < v && v < k) {
			h--;
			if (h == 0) {
				v = 0;
			}
		}
		if (v <= 0) {
			h = 0;
			v = 0;
		}
		x += v;
		if (s > 0) {
			s--;
		}
	}
	cout << x << "\n";
	return 0;
}