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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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    ll dp[n + 1][2] = {};
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    dp[1][0] = dp[1][1] = 1;
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    for (int i = 2; i <= n; ++i) {
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        dp[i][0] = dp[i - 1][0] + dp[i - 1][1];
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        dp[i][1] = dp[i - 1][0];
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    }
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    cout << dp[n][0] + dp[n][1];
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    return 0;
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}
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/*
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n = 3
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000
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001
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010
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011 - x
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100
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101
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110 - x
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111 - x
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dp[len][0]
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dp[len][1]
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0
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1
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dp[1][0] = 1
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dp[1][1] = 1
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00, 10
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01
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dp[2][0] = dp[1][0] + dp[1][1] = 1 + 1 = 2
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dp[2][1] = dp[1][0] = 1
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000, 100, 010
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001, 101
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dp[3][0] = dp[2][0] + dp[2][1] = 2 + 1 = 3 
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dp[3][1] = dp[2][0] = 2
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*/
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