POJ 3518

#include<string.h>#include<cstdio>#include<cstdlib>#include<iostream>#include<vector> #include<set>#include<cmath>#include<algorithm>using namespace std;const int maxn=1299710;bool u[maxn];int ans[maxn];void prepare(){int i,j,k;for(i=2;i<maxn;i++)u[i]=1;for(i=2;i<maxn;i++)if(u[i])for(j=2;j*i<maxn;j++)u[i*j]=0;for(i=2;i<maxn;i++)if(!u[i]){j=i;while(j<maxn&&!u[j])j++;j--;for(k=i;k<=j;k++)ans[k]=j-i+2;i=j;}else ans[i]=0;}int main(){int i,j,k;prepare();while(scanf("%d",&k)>0&&k){printf("%d\n",ans[k]);}}