UVa 1644

#include <bits/stdc++.h>
// #include <iostream>
// #include <cstdio>
// #include <cstdlib>
// #include <algorithm>
// #include <cmath>
// #include <vector>
// #include <set>
// #include <map>
// #include <queue>
// #include <ctime>
// #include <cassert>
// #include <complex>
// #include <string>
// #include <cstring>
// #include <queue>
// #include <bitset>

using namespace std;

// #pragma GCC optimize("Ofast,no-stack-protector")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
// #pragma GCC optimize("unroll-loops")


#define ll              long long int
#define vi              vector< int >
#define vll             vector< ll >

#define sc              scanf
#define pf              printf
#define cspf(i)         pf("Case #%d: ", i)
#define spc             pf(" ")
#define line            pf("\n")

#define ff              first
#define ss              second
#define mp              make_pair
#define pb              push_back
#define ppb             pop_back
#define tp(v,j)         get<j>(v)
#define Log(b,x)        (log(x)/log(b))

#define FOR(i,x,y)      for(int i = int(x); i < int(y); i++)
#define ROF(i,x,y)      for(int i = int(x)-1; i >= int(y); i--)
#define clr(arr,x)      memset(arr, x, sizeof arr)
#define vout(v,sz)      for(int w=0;w<sz;w++){if(w) spc; cout<<v[w];}
#define all(v)          v.begin(), v.end()
#define rall(v)         v.rbegin(), v.rend()
#define unq(v)          sort(all(v)),(v).resize(unique(all(v))-v.begin())
#define fastIO          ios::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr)

#define sc1(x)          sc("%d",&x)
#define sc2(x,y)        sc("%d %d", &x, &y)
#define sc3(x,y,z)      sc("%d %d %d", &x, &y, &z)
#define scl1(x)         sc("%lld",&x);
#define scl2(x,y)       sc("%lld %lld", &x, &y)
#define scf1(x)         sc("%lf",&x)
#define scf2(x,y)       sc("%lf %lf", &x, &y)

#define pf1(x)          pf("%d",x)
#define pf2(x,y)        pf("%d %d", x, y)
#define pf3(x,y,z)      pf("%d %d %d", x, y, z)
#define pfl1(x)         pf("%lld",x)
#define pfl2(x,y)       pf("%lld %lld",x,y)

#define MOD             (ll)(1e9+7)
#define MaxN            1300000
#define inf             0x3f3f3f3f
#define PI              acos(-1.0)  // 3.1415926535897932
#define eps             1e-9

template <class T> inline T bigMod(T p,T e,T M){T ret=1; for(;e>0;e>>=1){ if(e&1) ret=(ret*p)%M; p=(p*p)%M;} return (T)ret;}
template <class T> inline T modInverse(T a,T M){return bigMod(a,M-2,M);}
template <class T> inline T gcd(T a,T b){if(b==0)return a;return gcd(b,a%b);}
template <class T> inline T lcm(T a,T b) {a=abs(a);b=abs(b); return (a/gcd(a,b))*b;}
template <class T> inline T SQR(T a){return a*a;}

int dx[] = { 1,-1, 0, 0};                //graph moves
int dy[] = { 0, 0, 1,-1};               //graph moves
int kx[] = {-2,-2,-1,-1, 1, 1, 2, 2};  // Knights Move
int ky[] = {-1, 1,-2, 2,-2, 2,-1, 1}; // Knights Move

bool f[MaxN];
int val[MaxN];

void seive(){
    for(int i=4; i<MaxN; i+=2) f[i]=true;

    for(int i=3; i*i<MaxN; i+=2){
        if(f[i]) continue;
        for(int j=i*i; j<MaxN; j+=(2*i)) f[j]=true;
    }
}

void precalc(){
    int cnt=0;
    for(int i=4, j=3; i<MaxN; i++){
        cnt++;
        if(!f[i]){
            for(int k=j+1; k<i; k++){
                val[k]=cnt;
            }
            cnt=0;
            j=i;
        }
    }
}


int main(){

    #ifndef ONLINE_JUDGE
        clock_t tStart = clock();
        freopen("input.txt", "r", stdin);
        freopen("output.txt", "w", stdout);
    #endif

    seive();
    precalc();

    while(1){
        int n; sc1(n);
        if(!n) break;
        pf1(val[n]); line;
    }

    #ifndef ONLINE_JUDGE
        fprintf(stderr, "\n>> Runtime: %.10fs\n", (double) (clock() - tStart) / CLOCKS_PER_SEC);
    #endif

    return 0;
}