#include <bits/stdc++.h>
using namespace std;
struct TreeNode{
	TreeNode* left;
	TreeNode*right;
	int val;
	TreeNode(int val):left(nullptr),right(nullptr),val(val){};
};
void helper(TreeNode* root,vector<int>&in){
	if(root == nullptr)return;
   	
	helper(root->left,in);
	in.push_back(root->val);
	helper(root->right,in);
	
}
vector<int>inOrd(TreeNode* root){
	 vector<int>in;
	 helper(root,in);
	 return in;
}
TreeNode* buildTree(){
	int x;
	cin>>x;
	if(x==-1)return nullptr;
	TreeNode* root = new TreeNode(x);
	
	queue<TreeNode*>q;
	q.push(root);
	while(!q.empty()){
		auto u = q.front();
		q.pop();
		
		if(cin>>x && x!=-1){
			u->left = new TreeNode(x);
			q.push(u->left);
		}
		
		if(cin>>x && x!=-1){
			u->right = new TreeNode(x);
			q.push(u->right);
		}
	}
	return root;
}
int main() {
    TreeNode* root = buildTree();
    vector<int>ans = inOrd(root);
    for(int x : ans)cout<<x<<endl;
	return 0;
}