#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main(){
    int N; cin>>N;
    vector<bool> prime(200100,true);
    prime[0] = prime[1] = false;
    vector<int> dp(200010,-1);
    dp[0] = 0;
    
    for(int i=2;i<=200000;++i){
        if(prime[i]){
            for(int j=2;j*i<=200000;++j){
                prime[i*j] = false;
            }
        }
    }

    for(int i=2;i<=N;++i){
        if(prime[i]){
            for(int j=N;j>=0;--j){
                if(dp[j] == -1 || 200000 < i+j) continue;
                dp[i+j] = max(dp[i+j],dp[j]+1);
            }
        }
    }

    cout<<dp[N]<<endl;
}