#include "iostream"
#include "cmath"
using namespace std;
bool is_primer (int n)//判断是否为素数!
{
if (n == 1)
return 0;
int m;
m = floor(sqrt(double (n)) + 0.5);
for (int i = 2; i <= m; i++)
if (n % i == 0)
return 0;
return 1;
}
int main()
{
int InputNum;
int FrontPrimer, BackPrimer;
while (cin >> InputNum && InputNum)
{
int length = 0;
if (is_primer(InputNum))//如果是素数,直接输出结果为0
cout << length << endl;
else
{
for (int i = InputNum; i >= 2; i--)//求这个数的前一个素数!
if (is_primer(i))
{
FrontPrimer = i;
break;
}
for (int i = InputNum; ; i++)//求这个数的后一个素数!
if (is_primer(i))
{
BackPrimer = i;
break;
}
length = BackPrimer - FrontPrimer;//长度为两个素数的差!
cout << length << endl;
}
}
}