#include<iostream>
using namespace std;
int prime(int n)
{
for(int i=2;i*i<=n;i++)
{
if(n%i==0)
{
return false;
}
}
return true;
}
int main()
{
int t;
cin>>t;
while(t--)
{
int x,y;
cin>>x>>y;
int sum=x+y;
int flag=1;
int i=1;
while(flag)
{
int a=prime(x+y+i);
if(a==1)
{
cout<<i<<"\n";
flag=0;
}
i++;
}
}
return 0;
}
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