UVA- Prime gap

#include<bits/stdc++.h>
using namespace std;
#define fastread()(ios_base::sync_with_stdio(false),cin.tie(NULL));
typedef long long ll;
#define N 12997800
#define m 3606
vector<int>prime;
vector<bool>isprime(N,true);
void primefactor()
{
    int pos=1,i,j;
    for(i=4;i<=N;i+=2)isprime[i]=false;
    prime.push_back(2);
    for(i=3;i<=m;i+=2)
    {
        if(isprime[i])
        {
            for(j=i*i;j<N;j+=i)
            {
                isprime[j]=0;
            }
            prime.push_back(i);
        }
    }
    for(i=m;i<=N;i++)
    {
        if(isprime[i])prime.push_back(i);
    }
}
int main()
{
    fastread();
    primefactor();
    int n,i;
    while(scanf("%d",&n))
    {
        if(n==0)break;
        else if(isprime[n])printf("0\n");
        else
        {
            i=upper_bound(prime.begin(),prime.end(),n)-prime.begin();
            printf("%d\n",prime[i]-prime[i-1]);
        }
    }
    return 0;
}