正解写完后写完后，随便写了个 $unordered\_multiset$，结果本地样例过了，ide T了，CF W了。。。刚刚自学的这玩意儿 (((

#include<cstdio>
#include<cctype>
#include<unordered_set>
std::unordered_multiset<char>str;
char c;
int main()
{
 	while(scanf("%c",&c),islower(c))
	{
		str.insert(c);
	}
	printf("%llu",std::min(str.size(),str.count('a')*2-1));
	return 0;
}