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#include<bits/stdc++.h>
#define ll long long
using namespace std;
ll a[100005];
int main(){
ll n;
cin >> n;
a[0] = 0;
a[1] = 1;
a[2] = 2;
for(int i = 3;i <= n;i++) a[i] = a[i-1] + a[i-2];
cout << a[n];
return 0;
}