You are given an integer n. Your task is to return the nth Fibonacci number using an efficient approach.
The Fibonacci sequence starts with F(1) = 0 and F(2) = 1. Each subsequent term is calculated as the sum of the previous two terms.
Input: n = 10
Output: 34
Explanation: The 10th term of the Fibonacci sequence 0, 1, 1, 2, 3, 5, 8, 13, 21, 34 is 34.
Input: n = 1
Output: 0
Explanation: The first Fibonacci number is 0.
Input: n = 5
Output: 3
Explanation: The 5th term of the Fibonacci sequence is 3.
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