E0019.cpp

// Problem Code: POTATOES

#include <iostream>
#include <vector>

using namespace std;

void sieveOfEratosthenes(vector<int> &primes){
	int i, j, size = 2003;
	vector<bool> isPrime(size+1, true);
	for(i=2 ; i*i<=size ; i++)
		if(isPrime[i]){
			primes.push_back(i);
			for(j=2*i ; j<=size ; j+=i)
				isPrime[j] = false;
		}
	for(; i<=size ; i++)
		if(isPrime[i])
			primes.push_back(i);
}

int farmerFeb(int x, int y, vector<int> &primes){
	vector<int>::iterator it = lower_bound(primes.begin(), primes.end(), x+y+1);
	return *it-(x+y);
}

int main()
{
	
	int T, x, y;
	vector<int> primes;
	sieveOfEratosthenes(primes);
	cin >> T;
	for(int i=1 ; i<=T ; i++){
		cin >> x >> y;
		cout << farmerFeb(x, y, primes) << endl;
	}
	return 0;
}