运行以下二分查找程序，输出为（）

def find(x):
    a = [1,3,5,7,9]
    l, r = 0, 4
    while l <= r:
        mid = (l + r) // 2
        if a[mid] == x:
            return mid
        elif a[mid] < x:
            l = mid + 1
        else:
            r = mid - 1
    return -1
print(find(5))