http://codeforces.com/gym/101982/attachments/download/7897/20182019-acmicpc-pacific-northwest-regional-contest-div-1-en.pdf

#include<bits/stdc++.h>
using namespace std;
#define ll long long

bool notp[1000005];
int prime[1000005];

int x[1000005];


int cnt=0;
void init(){
    notp[1]=1;
    for(int i=2;i<=1000000;i++){
        if(notp[i])
            ;
        else{
            prime[cnt++]=i;
            for(int j=i+i;j<=1000000;j+=i)
                notp[j]=1;
        }
    }
}

int main(){
    init();
    for(int i=4;i<=1000000;i+=2){
        for(int j=0;j<cnt&&prime[j]<=i;j++){
            if(notp[i-prime[j]]==0){
                /*if(j>100)
                    cout<<i<<"!"<<j<<endl;*/
                x[i]=prime[j];
                break;
            }
        }
    }

    int X;
    int ans=0;
    while(cin>>X){
        ans=0;
        while(X>=4){
            X=X-2*x[X];
            ans++;
        }
        cout<<ans<<endl;
    }


}
 

2019-3-24