#include <iostream>
#include <sstream>
using namespace std;
int f[5005];
long long temp(string A, string B)//高精A+B 
{
	long long a[5005], b[5005], c[5005], len = max(A.length(), B.length());
	for(int i = A.length() - 1, j = 1; i >= 0; i--, j++) a[j] = A[i] - '0';
	for(int i = B.length() - 1, j = 1; i >= 0; i--, j++) b[j] = B[i] - '0';
	for(int i = 1; i <= len; i++)
	{
		c[i] += a[i] + b[i];
		c[i + 1] = c[i] / 10;
		c[i] %= 10;
	}
	if(c[len + 1]) len++;
	for(int i = len; i >= 1; i--) 
	{
    	return c[i];
	}
}
string tostr(long long value)//long long 转 string 
{
	stringstream ss;
    ss << value;
    string str = ss.str();
    return str;
}
int main()
{
	int n;
	cin >> n;
	f[0] = 0;
	f[1] = 1;
	f[2] = 2;
	for(int i = 3; i <= n; i++)
	{
		f[i] = temp(tostr(f[i - 1]), tostr(f[i - 2]));
	}
	cout << f[n];
	return 0;
} 

20pts，求助。