C++ :

#include <iostream>
using namespace std;
int n, a[25];
int main() {
	a[1] = 1;
	a[2] = 2;
	for (int i = 3; i <= 25; i++) {
		a[i] = a[i - 1] + a[i - 2];
	}
	cin>>n;	
	cout<<a[n]<<endl;
}

Python :

# coding=utf-8
n=int(input())
a=[0]*21
a[1]=1
a[2]=2
for i in range(3,21):
    a[i]=a[i-1]+a[i-2]
print(a[n])