6-175 Let $f$ be a function from $R \rightarrow R$, and
(1) For any $x, y \in R$,
$$f(x)+f(y)+1 \geqslant f(x+y) \geqslant f(x)+f(y)$$
(2) For any $x \in[0,1)$, $f(0) \geqslant f(x)$;
(3) $-f(-1)=f(1)=1$.

Find all functions $f$ that satisfy the conditions.