// Skarpetki
package Bronze_IV_4;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
public class Ex26742 {
public static void main(String[] args) throws IOException {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
char[] socks = br.readLine().toCharArray();
int whiteCount = 0;
int blackCount = 0;
for (char c : socks) {
if (c == 'B') {
whiteCount++;
} else {
blackCount++;
}
}
int pair = whiteCount / 2 + blackCount / 2;
System.out.println(pair);
}
}