Problem Statement

An altar enshrines $N$ stones arranged in a row from left to right. The color of the $i$-th stone from the left $(1 \leq i \leq N)$ is given to you as a character $c_i$; R stands for red and W stands for white.

You can do the following two kinds of operations any number of times in any order:

• Choose two stones (not necessarily adjacent) and swap them.
• Choose one stone and change its color (from red to white and vice versa).

According to a fortune-teller, a white stone placed to the immediate left of a red stone will bring a disaster. At least how many operations are needed to reach a situation without such a white stone?

Constraints

• $2 \leq N \leq 200000$
• $c_i$ is R or W.

Input

Input is given from Standard Input in the following format:

N
c_{1}c_{2}...c_{N}

Output

Print an integer representing the minimum number of operations needed.