Baekjoon #18258

#include<bits/stdc++.h>

using namespace std;

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);

    int T; cin >> T;
    queue<int> Q;
    while(T--) {
        string cmd; cin >> cmd;
        if(cmd == "push") {
            int X; cin >> X;
            Q.push(X);
        }
        else if(cmd == "pop") {
            if(Q.empty()) cout << -1 << '\n';
            else {
                cout << Q.front() << '\n';
                Q.pop();
            }
        }
        else if(cmd == "size") {
            cout << (int)Q.size() << '\n';
        }
        else if(cmd == "empty") {
            cout << Q.empty() << '\n';
        }
        else if(cmd == "front") {
            if(Q.empty()) cout << -1 << '\n';
            else cout << Q.front() << '\n';
        }
        else if(cmd == "back") {
            if(Q.empty()) cout << -1 << '\n';
            else cout << Q.back() << '\n';
        }
    }

    return 0;
}

import java.util.*;
import java.io.*;

public class Main {

	public static void main(String[] args) throws IOException {
        FastReader input = new FastReader();
        
        Deque<Integer> deque = new ArrayDeque<>(); // back명령어를 사용하기 위해 deque이용
        StringBuilder sb = new StringBuilder();
        
        int N = input.nextInt();
		while(N-->0) {
			String cmd = input.next();			
			switch (cmd) {
				case "push":
					int X = input.nextInt();
					deque.add(X);
					break;
				case "pop":
					if(deque.isEmpty()) sb.append(-1+"\n");
					else sb.append(deque.poll()+"\n");
					break;
				case "size":
					sb.append(deque.size()+"\n");
					break;
				case "empty":
					if(deque.isEmpty()) sb.append(1+"\n");
					else sb.append(0+"\n");
					break;
				case "front":
					if(deque.isEmpty()) sb.append(-1+"\n");
					else sb.append(deque.peekFirst()+"\n");
					break;
				case "back":
					if(deque.isEmpty()) sb.append(-1+"\n");
					else sb.append(deque.peekLast()+"\n");
			}
		}
        System.out.print(sb);
	}
	
    static class FastReader {
        BufferedReader br;
        StringTokenizer st;

        public FastReader() throws IOException{
        	br = new BufferedReader(new InputStreamReader(System.in)); //백준 제출 시 주석해제를 하면 됩니다.
        	// br = new BufferedReader(new FileReader("input.txt")); //IDE 실행 시 주석해제를 하면 됩니다.
        }

        String next() {
            while(st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                }
                catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }

        int nextInt() { return Integer.parseInt(next()); }
        long nextLong() { return Long.parseLong(next()); }
        String nextLine() {
            String str = "";
            try {
                str = br.readLine();
            }
            catch (IOException e) {
                e.printStackTrace();
            }
            return str;
        }
    }
}

import sys
from collections import deque
def input():
    return sys.stdin.readline().rstrip()

queue = deque()
N = int(input())
for i in range(N):
    com = input().split()
    if com[0] == 'push':
        queue.append(com[1])
    elif com[0] == 'pop':
        if queue:
            print(queue.popleft())
        else:
            print(-1)
    elif com[0] == 'size':
        print(len(queue))
    elif com[0] == 'front':
        if queue:
            print(queue[0])
        else:
            print(-1)
    elif com[0] == 'back':
        if queue:
            print(queue[-1])
        else:
            print(-1)
    elif com[0] == 'empty':
        if not queue:
            print(1)
        else:
            print(0)