m = {}
def f(x):
    if x == 1:
        return 2
    if x == 2:
        return 3
    if m.get(x, 0) != 0:
        return m[x]
    m[x] = f(x - 1) + f(x - 2)
    return m[x]

print(f(int(input())))

斐波那契数列更快的解法：矩阵快速幂（逃