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#include <bits/stdc++.h>
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#define ll long long
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using namespace std;
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int main() {
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    std::ios_base::sync_with_stdio(false);
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    cin.tie(0);
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    cout.tie(0);
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    int n;
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    cin >> n;
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    if (n == 1) {
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        cout << 1;
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        return 0;
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    } else if (n == 2) {
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        cout << 2;
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        return 0;
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    }
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    ll dp[n + 1] = {};
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    dp[n] = 1;
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    dp[n - 1] = 1;
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    dp[n - 2] = dp[n - 1] + dp[n];
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    for (int i = n - 3; i >= 0; i--) {
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        dp[i] = dp[i + 1] + dp[i + 2] + dp[i + 3];
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    }
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    cout << dp[0];
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    return 0;
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}