#include <iostream>
#include<stdio.h>
#include<vector>
using namespace std;
int main() {
// your code goes here
int T;
scanf("%d",&T);
int prime[10000]={0};
prime[0] = 1;
prime[1] = 1;
for(int i = 2 ;i<10000;i++)
{
if(prime[i] == 0)
{
for(int j = 2 ;i*j<10000;j++)
prime[i*j] = 1;
}
}
while(T--)
{
int a,b;
scanf("%d%d",&a,&b);
for(int i = 1;i<10000;i++)
{
if(prime[i+a+b] == 0)
{
printf("%d\n",i);
break;
}
}
}
return 0;
}
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