#include <iostream>

int calc_fib(int n) {
    if (n <= 1)
        return n;

    return calc_fib(n - 1) + calc_fib(n - 2);
}

int fib(int n)
{
    int a, b, c, i;

    if (n <= 1)
        return n;

    a = 0; b = 1;
    for(i = 1; i < n; i++) {
        c = a + b;
        a = b;
        b = c;
    }

    return b;
}

int main() {
    int n = 0;
    std::cin >> n;

    std::cout << fib(n) << '\n';
    return 0;
}