Iron, Magnet and Wall

Chef loves to play with iron (Fe) and magnets (Ma). He took a row of $N$ cells (numbered $1$ through $N$) and placed some objects in some of these cells. You are given a string $S$ with length $N$ describing them; for each valid $i$, the $i$-th character of $S$ is one of the following: - 'I' if the $i$-th cell contains a piece of iron - 'M' if the $i$-th cell contains a magnet - '_' if the $i$-