#!/bin/python3

import sys
import math
def howManyGames(p, d, m, s):

        if((p-m)%d==0):
            n=((m-p)/-d) +1
            
                
        elif ((p-m)%d!=0):
            n=((((p-m)%d+m)-p)/-d)+1
            
        sum=((n/2)*(p+m))
        if(sum<=s):
            k=n+(s-sum)//m
        
        return(k-1)
        
        
        
    # Return the number of games you can buy

if __name__ == "__main__":
    p, d, m, s = input().strip().split(' ')
    p, d, m, s = [int(p), int(d), int(m), int(s)]
    answer = howManyGames(p, d, m, s)
    print(answer)