素数筛没什么好说的
#include<cstdio>
#include<vector>
#include<cstring>
#define REP(i, a, b) for(int i = (a); i < (b); i++)
using namespace std;const int MAXN = 2123456;
bool is_prime[MAXN];
vector<int> prime;void init()
{memset(is_prime, true, sizeof(is_prime));is_prime[0] = is_prime[1] = false;REP(i, 2, MAXN){if(is_prime[i]) prime.push_back(i);REP(j, 0, prime.size()){if(i * prime[j] >= MAXN) break;is_prime[i * prime[j]] = false;if(i % prime[j] == 0) break;}}
}int solve(int x)
{if(is_prime[x]) return 0;int i, j;for(i = x; !is_prime[i]; i--);for(j = x; !is_prime[j]; j++);return j - i;
}int main()
{init();int n;while(~scanf("%d", &n) && n) printf("%d\n", solve(n));return 0;
}