#include <iostream>
#include <stack>
#include <cmath>
#include <vector>
#include <string.h>
#include <queue>
#include <stdio.h>
#include <iomanip>
#include <cstdio>
#include <algorithm>
#define int long long
using namespace std;
const int N = 1e5 + 10;
const int INF = 0x3f3f3f3f;
/*int f(int x)
{
	if(x == 1 || x == 2)
	{
		return 1;
	}
	return f(x - 1) + f(x - 2);
}
signed main()
{
	int n;
	cin >> n;
	cout << f(n) << endl; 
}*/
int n;
signed main()
{
	cin >> n;
	int a, b, c;
	a = b = c = 1;
	for(int i = 3; i <= n; i++)
	{
		c = a + b;
		a = b;
		b = c;
	}
	cout << c << endl;
}

数据可能有一点点大

#include <iostream>
#include <math.h>
#include <iomanip>
using namespace std;
int main()
{
    int n;
    cin >> n;
    int a,b,c;
    a = b = c = 1;
    for(int i = 3; i <= n ; ++i)
    {
    	c = a + b;
    	a = b;
    	b = c;
	}
	cout << c << endl;   
}

#include <stdio.h>
#include <iostream>
using namespace std;
int main()
{
    int n;
    cin >> n;
    int a,b,c;
    a = b = c = 1;
    for (int i = 3;i<= n; i++)
    {
        c = a + b;
        a = b;
        b = c;
    }
    cout << c << endl;
}