procedure main()
    nws := ~' \t'; stk := []
    while read() ? {
        while (not pos(0)) &
              push(stk, real(x := (tab(upto(nws)),tab(many(nws)))\1) |
                        (x:=proc(x,2),a:=pop(stk),x(pop(stk),a)))
        write(stk[1] | "error")
        }
end