program gff;
Var q,d,n,x,t,w,i:Integer;
s:Integer;
Procedure Degree(x,y:Integer; Var st : Integer);
Var
i:integer;
Begin
st:=1;
For i:=1 To y Do st:=st*x;
End;
Begin
Writeln('vedite isxodnii dannye');
Readln(q,d,n,x);
s:=0;
t:=n;
For i:=1 To n+1 Do Begin
Degree (x,t,w); s:=s+q*w;
Dec(t);
Inc(q,d);
End;
Writeln('resyltat',s);
readln; End.