rt，说明/提示中的 $\LaTeX$ 整个炸掉了，因此我修复了一下：

Here are some examples for understanding carries:

So $f(7,4)=1$,$f(5,1)=1$ and $f(5,3)=3$.

In the first test case, all the pairs meeting the constraints are

$(1,1),(1,5),(2,2),(2,3),(3,2),(4,4),(4,5),(4,6),(4,7),(5,1),(5,4),(5,6),(6,4),(6,5),(7,4)$.

原码：

Here are some examples for understanding *carries*:

$$
\begin{aligned} 
&\begin{array}{r} 
~1~~ 1~~ 1~~ \\
+~_{1}1~~ 0~~ 0~~ \\
\hline
1~~ 0~~ 1~~ 1~~ 
\end{array}
&\begin{array}{r}
~1~~ 0~~ 1~~ \\
+~_{~}0~~ 0_{1~} 1~~ \\
\hline
0~~ 1~~ 1~~ 0~~ 
\end{array} 
& &\begin{array}{r}
~1~~ 0~~ 1~~ \\
+~_{1}0_{1} 1_{1~} 1~~ \\
\hline
1~~ 0~~ 0~~ 0~~ 
\end{array} 
\end{aligned}
$$

So $f(7,4)=1$,$f(5,1)=1$ and $f(5,3)=3$.

In the first test case, all the pairs meeting the constraints are

$(1,1),(1,5),(2,2),(2,3),(3,2),(4,4),(4,5),(4,6),(4,7),(5,1),(5,4),(5,6),(6,4),(6,5),(7,4)$.