/**
 * 
 * @param n int整型 
 * @return int整型
 */
int Fibonacci(int n) {
    // write code here
    if(n <= 2) return 1;
    int a = 1, b = 1, c = 2;
    for(int i = 3; i <= n; i++){
        c = a + b;
        a = b;
        b = c;
    }
    return c;
}