#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;


int main() {
int p,d,m,s;
    cin>>p>>d>>m>>s;
    int n=(p-m)/d;
    int sum=((n+1)*(2*p-n*d))/2;
    if(sum<=s)
    {
        cout<<n+1+(s-sum)/m;
    }
    else 
    {
        int count=0;
        int sum=0;
        int x=p;
        while(s>=x)
        {
          count++;
            s=s-x;
            x=x-d;
            if(x<m) x=m;
        }
        cout<<count;
        
    }
    return 0;
}