BACTHANG1 C++

#include <bits/stdc++.h>
using namespace std;
int n;
long long f[55];

void giai(){
    f[2]=1;f[3]=2;
    for(int i=4;i<=n;i++)
        f[i]=f[i-1]+f[i-2];
    cout<<f[n];
}

int main(){
    freopen("bacthang1.inp", "r", stdin);
    freopen("bacthang1.out", "w", stdout);
    cin>>n;
    giai();
}