#include <iostream>
using namespace std;
void count(int n){
int dp[n+1];
dp[0] = 0;
dp[1] = 1;
dp[2] = 2;
dp[3] = 4;
for(int i=4;i<=n;i++){
dp[i] = dp[i-1]+dp[i-2]+dp[i-3];
}
cout<<dp[n];
}
int main() {
int n;
cin>>n;
count(n);
// your code goes here
return 0;
}
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