A1009 | No Time to Paint--Silver

Bessie has recently received a painting set, and she wants to paint the long
fence at one end of her pasture. The fence consists of $N$ consecutive 1-meter
segments ($1\le N\le 10^5$). Bessie has 26 different colors available, which
she labels with the letters 'A' through 'Z' in increasing order of darkness
('A' is a very light color, and 'Z' is very dark). She can therefore describe
the desired color she wants to paint each fence segment as a length-$N$ string
where each character is a letter.
Initially, all fence segments are uncolored. Bessie can color any contiguous
range of segments with a single color in a single brush stroke as long as she
never paints a lighter color over a darker color (she can only paint darker
colors over lighter colors).
For example, an initially uncolored segment of length four can be colored as
follows:
.... -> BBB. -> BBLL -> BQQL
Running short on time, Bessie thinks she may need to leave some consecutive
range of fence segments unpainted! Currently, she is considering $Q$ candidate
ranges ($1\le Q\le 10^5$), each described by by two integers $(a,b)$ with $1
\leq a \leq b \leq N$ giving the indices of endpoints of the range $a \ldots
b$ of segments to be left unpainted.
For each candidate range, what is the minimum number of strokes needed to
paint every fence segment outside those in the range with its desired color
while leaving all fence segments inside the range uncolored? Note that Bessie
does not actually do any painting during this process, so the answers for each
candidate range are independent.

The first line contains $N$ and $Q$.
The next line contains a string of length $N$ characters representing the
desired color for each fence segment.
The next $Q$ lines each contain two space-separated integers $a$ and $b$
representing a candidate range to possibly leave unpainted.

For each of the $Q$ candidates, output the answer on a new line.