#include <stdio.h>
#include <string.h>

int stack[100];
int top = -1;

void push(int x){
    stack[++top] = x;
}

void pop(){
   top--;
}

int main(){
    char arr[] = "ABBCCDA";
    
    for (int i = 0;i < 7;i++) {
        if (stack[top] == arr[i]) {
            pop();
        }else{
        push(arr[i]);
        }
    }
    
    for (int i = 0; i <= top; i++) {
        printf("%c", stack[i]);
    }
}