import java.io.*;
import java.util.*;
import java.util.function.*;
import java.util.stream.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        int n = sc.nextInt();
        String result = IntStream.range(0, n)
            .mapToObj(i -> String.valueOf(solve(sc.nextInt(), sc.nextInt())))
            .collect(Collectors.joining("\n"));
        System.out.println(result);
    }
    
    static int solve(int a, int b) {
        if (a == b) {
            return 0;
        } else {
            int diff = b - a;
            int ans = Integer.MAX_VALUE;
            for (int i = 1; i <= Math.sqrt(diff); i++) {
                if (diff % i == 0) {
                    if (i >= a) {
                        ans = Math.min(i, ans);
                    }
                    if (diff / i >= a) {
                        ans = Math.min(diff / i, ans);
                    }
                }
            }
            return ans == Integer.MAX_VALUE ? -1 : ans - a;
        }
    }
}
class Scanner {
    BufferedReader br;
    StringTokenizer st = new StringTokenizer("");
    StringBuilder sb = new StringBuilder();
    
    public Scanner() {
        try {
            br = new BufferedReader(new InputStreamReader(System.in));
        } catch (Exception e) {
            
        }
    }
    
    public int nextInt() {
        return Integer.parseInt(next());
    }
    
    public long nextLong() {
        return Long.parseLong(next());
    }
    
    public double nextDouble() {
        return Double.parseDouble(next());
    }
    
    public String next() {
        try {
            while (!st.hasMoreTokens()) {
                st = new StringTokenizer(br.readLine());
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            return st.nextToken();
        }
    }
    
}