import java.util.Scanner;
class Main{
public static Scanner scn = new Scanner(System.in);
public static void main(String[] args){
sanketStrings();
}
public static void sanketStrings(){
int k = scn.nextInt();
String str = scn.next();
int count_a = 0;
int count_b = 0;
for(int i = 0;i< str.length();i++){
if(str.charAt(i)=='a'){
count_a++;
}else{
count_b++;
}
}
int perfect = 0;
if(count_a>count_b){
perfect=count_a;
if(count_b<=k){
perfect = perfect + count_b;
}else{
perfect = perfect + k;
}
}else{
perfect = count_b;
if(count_a<=k){
perfect = perfect + count_a;
}else{
perfect = perfect + k;
}
}
System.out.println(perfect);
}
}
