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 PyPy3 (2.4.0) (judge verdict: Accepted)

from math import sqrt

def digitsum(b,n):
    if b>n:
        return n
    else:
        return digitsum(b,n//b)+n%b

def divisors(M):#Mの約数列 O(n^(0.5+e))
    import math
    d=[]
    i=1
    while math.sqrt(M)>=i:
        if M%i==0:
            d.append(i)
            if i**2!=M:
                d.append(M//i)
        i=i+1
    d.sort()
    return d

n=int(input());s=int(input())
if 0>n-s:
    print(-1)
else:
    m=int(sqrt(n))
    for b in range(2,m+1):
        if digitsum(b,n)==s:
            print(b)
            break
    else:
        d=divisors(n-s)
        for i in d:
            if i>=m and s>=(n-s)//i and i+1>(s-(n-s)//i):
                print(i+1)
                break
        else:
            if n==s:
                print(n+1)
            else:
                print(-1)