Problem:
Score : 500 points

Problem Statement

For integers b (b \geq 2) and n (n \geq 1), let the function f(b,n) be defined as follows:

f(b,n) = n, when n < b

f(b,n) = f(b,\,{\rm floor}(n / b)) + (n \ {\rm mod} \ b), when n \geq b

Here, {\rm floor}(n / b) denotes the largest integer not exceeding n / b,
and n \ {\rm mod} \ b denotes the remainder of n divided by b.

Less formally, f(b,n) is equal to the sum of the digits of n written in base b.
For example, the following hold:

f(10,\,87654)=8+7+6+5+4=30

f(100,\,87654)=8+76+54=138

You are given integers n and s.
Determine if there exists an integer b (b \geq 2) such that f(b,n)=s.
If the answer is positive, also find the smallest such b.

Constraints

1 \leq n \leq 10^{11}

1 \leq s \leq 10^{11}

n,\,s are integers.

Input

The input is given from Standard Input in the following format:

n
s

Output

If there exists an integer b (b \geq 2) such that f(b,n)=s, print the smallest such b.
If such b does not exist, print -1 instead.

Sample Input 1

87654
30

Sample Output 1

10

Sample Input 2

87654
138

Sample Output 2

100

Sample Input 3

87654
45678

Sample Output 3

-1

Sample Input 4

31415926535
1

Sample Output 4

31415926535

Sample Input 5

1
31415926535

Sample Output 5

-1

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Wrong Answer)

import java.util.*;

public class Main {
  public static void main(String[] args) throws Exception {
    Scanner scanner = new Scanner(System.in);
    long n = scanner.nextLong();
    long s = scanner.nextLong();
    if(n < s) {
      System.out.println(-1);
    } else if(n == s) {
      System.out.println(n+1);
    } else {
      boolean ok = false;
      double sqrtN = Math.sqrt(n);
      for(long i=2;i<sqrtN;i++) {
        if(f(i, n) == s) {
          System.out.println(i);
          ok = true;
          break;
        }
      }
      if(!ok) {
        for(long p=1;p<=sqrtN;p++) {
          if((n-s)%p != 0) continue;
          long b = (n-2)/p+1;
          if(f(b, n) == s) {
            System.out.println(b);
            ok = true;
            break;
          }
        }
      }
      if(!ok) System.out.println(-1);
    }

    scanner.close();
  }

  private static long f(long b, long n) {
    long ret = (n<b)? n : (f(b, (long)Math.floor(n/b))+n%b);
    return ret;
  }
}