#include <stdio.h>
#include <cmath>
bool p(int n){
	if(!(n^2))return 1;
	if(!(n%2))return 0;
	int q = (int)sqrt(n+0.0);
	for(int i=3; i<=q;i+=2){
		if(!(n%i))return 0;
	}
	return 1;
}
int main() {
	int n;
	while(1){
		scanf("%d",&n);
		if(!n)break;
		if(p(n))printf("0\n");
		else{
			int s=n-1,x=n+1;
			for(;!p(s);s--);
			for(;!p(x);x++);
			printf("%d\n",x-s);
		}
	}
	return 0;
}