C++ :

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

int n, cnt;
char ans[1000009];
string s;

void dfs(int u) {
	if (u >= n) return;
	
	ans[u] = s[cnt ++];
	dfs(2 * u + 1);
	dfs(2 * u + 2);
}

int main() {
	while (cin >> s) {
		n = s.size();
		cnt = 0;
		memset(ans, 0, sizeof(ans));
		
		dfs(0);
		for (int i = 0; i < n; i ++) cout << ans[i];
		cout << endl;
	}

	return 0;
}