// editor1
#include<stdio.h>
#include<stdlib.h>
typedef struct TreeNode{
    int data;
    struct TreeNode *left;
    struct TreeNode *right;
}Node;
Node*root=NULL;
Node *create(int num){
    Node *newnode=(Node*)malloc(sizeof(Node));
    newnode->data=num;
    newnode->left=newnode->right=NULL;
    return newnode;
}
Node*buildTree(int arr[],int n){
    if(n<=0) return NULL;
    
    Node* root = create(arr[0]);
    Node* queue[20];
    int front=0,rear=0;
    queue[rear++]=root;
    int i=1;
    while(i<n){
        Node* curr = queue[front++];
        if(i<n){
            curr->left=create(arr[i++]);
            queue[rear++]=curr->left;
        }
        if(i<n){
            curr->right=create(arr[i++]);
            queue[rear++]=curr->right;
        }
    }
    return root;
}
void inorder(Node *root){
    Node* stack[20];
    int top=-1;
    Node* curr = root;
    while(Curr || top != -1){
        stack[++top]=curr;
        curr = curr->left;
    }
    curr = stack[top--];
    printf("%d",curr->data);
    curr=curr->right;
    
}

int main(){
    int n,arr[20];
    scanf("%d",&n);
    if(n<=0){
        printf("Invalid input");
        return 0;
    }
    for(int i=0;i<n;i++){
        scanf("%d",&arr[i]);
        if(arr[i]<=0){
            printf("Invalid input");
            return 0;
        }
    }
        Node* root=buildTree(arr,n);
    
    inorder(root);
    return 0;
}