#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;
    //cout<<n<<endl;
    int su=p*(n+1)-(n*(n+1)*d/2);
    if(su==s)
        cout<<n+1<<endl;
    else if(su<s)
    {
        cout<<(n+1+((s-su)/m))<<endl;
    }
    else
    {
        int i;
        for(i=n;i>=-1;i--)
        {
            int z=p*(i+1)-(i*(i+1)*d/2);
            if(z>s)
                continue;
            else
                break;
        }
        cout<<i+1<<endl;
    }
    return 0;
}