题面:

For example, if $t=\texttt{bababa}$ and $s_1=\texttt{ba}$ , $s_2=\texttt{aba}$ , you can get $t=\color{red}{\texttt{ba}}\texttt{baba}$ , $t=\texttt{b}\color{red}{\texttt{aba}}\texttt{ba}$ or $t=\texttt{bab}\color{red}{\texttt{aba}}$ in one step.

You want to color all the letters of the text $t$ in red. When you color a letter in red again, it stays red.

In the example above, three steps are enough:

• Let's color $t[2 \dots 4]=s_2=\texttt{aba}$ in red, we get $t=\texttt{b}\color{red}{\texttt{aba}}\texttt{ba}$ ;
• Let's color $t[1 \dots 2]=s_1=\texttt{ba}$ in red, we get $t=\color{red}{\texttt{baba}}\texttt{ba}$ ;
• Let's color $t[4 \dots 6]=s_2=\texttt{aba}$ in red, we get $t=\color{red}{\texttt{bababa}}$ .

正确题面:

For example, if $t=\texttt{bababa}$ and $s_1=\texttt{ba}$ , $s_2=\texttt{aba}$ , you can get $t=\color{red}{\texttt{ba}}\color{black}\texttt{baba}$ , $t=\texttt{b}\color{red}{\texttt{aba}}\color{black}\texttt{ba}$ or $t=\texttt{bab}\color{red}{\texttt{aba}}$ in one step.

You want to color all the letters of the text $t$ in red. When you color a letter in red again, it stays red.

In the example above, three steps are enough:

• Let's color $t[2 \dots 4]=s_2=\texttt{aba}$ in red, we get $t=\texttt{b}\color{red}\texttt{aba}\color{black}\texttt{ba}$ ;
• Let's color $t[1 \dots 2]=s_1=\texttt{ba}$ in red, we get $t=\color{red}\texttt{baba}\color{black}\texttt{ba}$ ;
• Let's color $t[4 \dots 6]=s_2=\texttt{aba}$ in red, we get $t=\color{red}{\texttt{bababa}}$ .

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