import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = Integer.parseInt(sc.nextLine());
		String str = sc.nextLine();
		sc.close();
		int[] x = new int[3];
		if (str.indexOf('G') != -1) {
			x[0] = count(str, 'G');
		}
		if (str.indexOf('S') != -1) {
			x[1] = count(str, 'S');
		}
		if (str.indexOf('B') != -1) {
			x[2] = count(str, 'B');
		}
		String[] str1 = new String[3];
		str1[0] = str.substring(0, x[0]);
		str1[1] = str.substring(x[0],x[0] + x[1]);
		str1[2] = str.substring(x[0] + x[1], x[0] + x[1] + x[2]);
		int count = 0;
		count(str1[0],'S');
		count(str1[0],'B');
		count(str1[1],'G');
		count(str1[1],'B');
		count(str1[2],'G');
		count = str1[0].length() - count(str1[0],'G');
		if(count(str1[0],'S') >= count(str1[1],'G')) {
			count += count(str1[1],'B');
		}else if(count(str1[0],'S') < count(str1[1],'G')) {
			count += count(str1[1],'B') + (count(str1[1],'G')-count(str1[0],'S'));
		}
		System.out.println(count);
	}

	public static int count(String s, char c) {
		int count = 0;
		for (int i = 0; i < s.length(); i++) {
			if (s.charAt(i) == c) {
				count++;
			}
		}
		return count;
	}
}
/**************************************************************
	Problem: 1578
	User: admin
	Language: Java
	Result: Accepted
	Time:3682 ms
	Memory:40292 kb
****************************************************************/