#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<algorithm>
#include<cmath>

#define md
#define ll long long
#define inf (int) 1e9
#define eps 1e-8
#define N
using namespace std;

int main()
{
    int x,y,w,n;
    while (scanf("%d%d%d%d",&x,&y,&w,&n)!=EOF)
    {
    ll t=0,now=0; int tot=1;
    while (tot<n)
    {
        t+=w;
        if (t>now*(x+y)+x) {  now++; t=now*(x+y)-w;}
          else tot++;
    }
    printf("%I64d\n",t);
    }
    return 0;
}