#include <iostream>
using namespace std;
int main() {
int N;
cin >> N;
int *a = new int [N + 1];
a[0] = 1;
a[1] = 1;
a[2] = 2;
for (int i = 3; i <= N + 1; i++)
a[i] = a[i - 1] + a[i - 2] + a[i - 3];
cout << a[N];
delete [] a;
return 0;
}
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