$$\large lev_{a,b}(i,j)=\begin{cases} max(i,j) & \text if min(i,j)=0 & min \begin{cases} lev _{a,b}(i-1,j)+1 & & lev _{a,b}(i,j-1)+1 & & lev _{a,b}(i-1,j-1)+1 & \end{cases} & \text otherwise. \end{cases}$$
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