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

#define MAX_QUEUE 45

int queue[MAX_QUEUE];
int front = 0;
int rear = -1;
int size = 0;

void enqueue(int slipNumber){
    if(size < MAX_QUEUE){
        rear = (rear + 1) % MAX_QUEUE;
        queue[rear] = slipNumber;
        size++;
    }
}

int dequeue(){
    if(size > 0){
        int patient = queue[front];
        front = (front + 1) % MAX_QUEUE;
        size--;
        return patient;
    }
    return -1;
}

int isEmpty(){
    return (size == 0);
    
}
int main(){
    char input[1000];
    char token[20];
    if(fgets(input,sizeof(input),stdin) == NULL){
        return 0;
    }
    int len = strlen(input);
    if(len > 0 && input[len-1] == '\n'){
        input[len-1] = '0';
    }
    char* ptr = strtok(input," ");
    while (ptr != NULL){
        if(strcmp(ptr,"B") == 0){
            if(isEmpty()){
                printf("Invalid input\n");
            }
            else{
                int calledpatient = dequeue();
                printf("%d\n",calledpatient);
            } 
            }else{
                printf("Invalid input\n");
                return 0;
            }
        }
        ptr = strtok(NULL," ");
    }
    return 0;
}