void backup(int *first)
{
    list *newfirst, *current, *currentprev, *oldcurrent;
    newfirst = (list*)malloc(sizeof(list));
    newfirst->undop = first;
    first->redop = newfirst;
    newfirst->val = first -> val;
    currentprev = newfirst;
    oldcurrent = first->next;
    while(oldcurrent!=NULL)
    {
        current = (list*)malloc(sizeof list);
        current -> val = oldcurrent -> val;
        current -> prev = currentprev;
        currentprev -> next = current;
        current->next = NULL;
        currentprev = current;
        oldcurrent = oldcurrent->next;
    }
    first = newfirst;
}