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- Project Euler #206: Concealed Square

# Project Euler #206: Concealed Square

# Project Euler #206: Concealed Square

_{This problem is a programming version of Problem 206 from projecteuler.net}

Find the unique positive integer whose square's decimal representation has the form

where are known single digits and are unknown single digits.

In other words, find the unique positive integer when given the every other digit of its square, starting with the first digit and including the last digit of the square.

**Input Format**

The first line contains a single positive integer denoting the number of the known digits.

The second line contains the list of single-space separated known digits.

It is guaranteed that the answer exists and it is unique.

**Constraints**

- as the number of known digits;
- The first known digit cannot be zero.

**Output Format**

On a single line print the answer.

**Sample Input 0**

```
3
8 7 6
```

**Sample Output 0**

```
286
```

**Explanation 0**

We need to find a positive integer whose square has the form where each is a single unknown digit. It is easy to validate that

Therefore the answer is .

**Sample Input 1**

```
4
1 2 3 4
```

**Sample Output 1**

```
1312
```

**Explanation 1**

We are looking for a positive integer whose square has the form . Given the total number of the digits in the square, we can speculate that the answer is an integer less than . Exhaustive search shows that

Therefore the answer is .