Segments and String

Given $N$ strings $st[ ]$ of length $M$ each and an integer K. The strings $st[i]$ satisfy a very special property : - $st[i + 1][j]$ != $st[i][j + 1]$, for all $i$, $j$ where ($1 \leq i < n$) and ($1 \leq j < m$). You choose $N$ segments [$L_{i}, R_{i}$] such that: - $1 = L_{1} \leq R_{1} = L_{2} \leq R_{2} = L_{3} \leq R_{3} ..... R_{N - 1} = L_{N} \leq R_{N} \leq M$. - $R_{1} - L_{1