var
  a, b, c, n, d: integer;
  sum: integer;
  b1, c1: real;

begin
  read(a, b, c);
  read(n);
  b1 := b;
  c1 := c;
  while (n > 0) do
  begin
    if (c1 / 28 < b1 / 7) and (n >= 28) and (((n + 27) div 28) * c <= (n * a)) then begin
      n := n - 28;
      sum := sum + c;
    end
    else  if ((n >= 7) and (((n + 6) div 7) * b < ((n + 27) div 28) * c)) then begin
      n := n - 7;
      sum := sum + b;
    end
    else
    if ((n >= 7) and (((n + 27) div 28) * c > (n * a)) and (((n + 6) div 7) * b > n * a)) then begin
      sum := sum + n * a;
      n := n - n
    end
    else begin
      if ((n <= 7) and (n * a < b)) then begin
        sum := sum + n * a;
        n := 0;
      end;
      if ((n <= 7) and (n * a > b)) then begin
        sum := sum + b;
        n := 0;
      end;
    end;
  end;
  writeln(sum);
end.