10845.cpp

#include<bits/stdc++.h>
using namespace std;

int main(void){
    ios::sync_with_stdio(0);
    cin.tie(0);
    int N;
    cin>>N;
    queue<int> Q;
    string op;
    while(N--){
        cin>>op;
        if(op=="push"){
            int input;
            cin>>input;
            Q.push(input);
        }else if(op=="pop"){
            if(Q.empty()) cout<<"-1\n";
            else
            { cout<<Q.front()<<"\n";
             Q.pop();
            }

        }else if(op=="back"){
            if(Q.empty()) cout<<"-1\n";
            else cout<<Q.back()<<"\n";

        }else if(op=="empty"){
               cout<<Q.empty()<<"\n";

        }else if(op=="front"){
             if(Q.empty()) cout<<"-1\n";
            else cout<<Q.front()<<"\n";
        }else if(op=="size"){
            cout<<Q.size()<<"\n";
        }
    }
}