#include <bits/stdc++.h>
using namespace std;
#define si(x)       scanf("%d",&x)
#define su(x)       scanf("%u",&x)
#define sll(x)      scanf("%lld",&x)
#define sl(x)       scanf("%ld",&x)     
#define sc(x)       scanf("%c",&x)
#define ss(x)       scanf("%s",x)
#define sf(x)       scanf("%f",&x)
#define slf(x)      scanf("%lf",&x)
#define pri(x)      printf("%d\n",x)
#define prl(x)      printf("%lld\n",x)
#define ll          long long int
#define mod(x,n)    (x+n)%n
#define pb          push_back
#define mp          make_pair
#define pii         pair<int,int>
#define pll         pair<long long,long long>
#define pil         pair<int,long long>
#define pli         pair<long long,int>
#define vi          vector<int>
#define vl          vector<long long>
#define all(a)      (a).begin(),(a).end()
#define F           first
#define S           second
#define sz(x)       (int)x.size()
#define M           1000000007


int main()
{
    //ios_base::sync_with_stdio(false);
    //cin.tie(NULL);
    //cout.tie(NULL);
    //freopen("input.txt","r",stdin);
    //freopen("output.txt","w",stdout);
	int p,d,m,s;
	si(p); si(d); si(m); si(s);
	int temp=(p-m)/d; temp++; 
	int temp1=((2*p-(temp-1)*d)*temp)/2;
	if((s-temp1)>=0){
		int cost=(s-temp1)/m;
		cost+=temp;
		pri(cost);
		return 0;
	}
	int cost=0,k=0;
	while(s>0){
		s-=(p-k*d);
		k++;
		cost++;
	}
    if(s<0) cost--;
	pri(cost);

   	return 0;
}