#include<bits/stdc++.h>
using namespace std;
int main(){
    int m;
    cin >> m;
    vector<int> v;
    string op;
    while(m--){
        cin >> op;
        if(op == "push_back"){
            int x;
            cin >> x;
            v.push_back(x);
        }else if(op == "size?"){
            cout << v.size() << endl;
        }else if(op == "back?"){
            if(v.empty()) cout << "error" << endl;
            else cout << v.back() << endl;
        }
    }
    return 0;
}