Simple Operations

You are given two strings $S$ and $R$. Each of these strings has length $N$. We want to make $S$ equal to $R$ by performing the following operation some number of times (possibly zero): - Choose two integers $a$ and $b$ such that $1 \le a \le b \le N$. - For each $i$ such that $a \le i \le b$, replace the $i$-th character of $S$ by the $i$-th character of $R$. Suppose that we make $S$ equal t