const int + 递推

#include<bits/stdc++.h>
using namespace std;
const int j=1e9+7,m=1e7;
int main(){
    int n;
    cin>>n;
    long long f[m]={0,1,2,4};
    for(int i=4;i<=n;i++){
        f[i]=(f[i-1]+f[i-2]+f[i-3])%j;
    }
    cout<<f[n];
    return 0;
}