#include <stdio.h>
#include <string.h>
char str[200001];

int main(){
   int cnt_a = 0, cnt_b = 0;
   int length = strlen(str);
   int ret = -1;
   for(int i = 0; i < length ; i++){
       if(str[i]=='a') cnt_a++;
       else             cnt_b++;
       if(cnt_a == cnt_b) ret = i;
   }
   printf("%d\n",ret+1);
}