#include<bits/stdc++.h>
using  namespace  std;

#define endl         "\n"
#define ll           long long
#define PI           acos(-1.0)
#define GCD(a,b)     __gcd(a , b)
#define LCM(a,b)     ((a/__gcd(a,b))*b)
#define READ(f)      freopen(f,"r",stdin)
#define WRITE(f)     freopen(f,"w",stdout)
#define test         cout<<"\n*************\n"
#define mem(arr,val) memset(arr,val,sizeof(arr))
#define precise(c)   fixed(cout);cout<<setprecision(c)
#define valid(x,y)   (x>=0 && x<row && y>=0 && y<column)
#define fast         ios_base :: sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)



int main(){

    ll n;
    cin>>n;

    ll dp[n+1];

    dp[0] = 0;
    dp[1] = 0;
    dp[2] = 1;

    for(ll i=3;i<=n;i++){
        dp[i] = dp[i-1] + dp[i-2];
    }

    cout<<dp[n]<<endl;
    
    return 0;
}