230718

# r = 10
# c = 10
#
# l = [[0 for i in range(c)] for j in range (r)]
#
# for i in range(10):
#     l[i] = list(map(int, input().split()))
#
# n = 1
# m = 1
# if l[n][m] == 2:
#     l[n][m] = 9
# else:
#     l[n][m] = 9
#     while True:
#         if l[n][m+1] == 0:
#             m += 1
#             l[n][m] = 9
#         elif l[n][m+1] == 2:
#             m += 1
#             l[n][m] = 9
#             break
#         else:
#             if l[n+1][m] == 0:
#                 n += 1
#                 l[n][m] = 9
#             elif l[n+1][m] == 2:
#                 n += 1
#                 l[n][m] = 9
#                 break
#             else:
#                 break
#
# l[n][m] = 9
# for i in range(len(l)):
#     for j in range(len(l[i])):
#         print(l[i][j], end = (' '))
#     print()

#
# n, k = list(map(int, input().split()))
#
# for i in range(n):
#     for j in range(n):
#         if i == 0 or j == 0 or i == n-1 or j == n-1 or ( i + j + 1) %k==0:
#             print('*', end='')
#         else:
#             print(' ', end='')
#     print()


# k = int(input())
# stk = [0]
#
# for i in range(k):
#     n = int(input())
#     if n != 0:
#         stk.append(n)
#     else:
#         stk.pop()
# print(sum(stk))

def sq(n):
    if n <= 3:
        return n
    return sq(n-1) + sq(n-2)

n = int(input())
print(sq(n))