def linear_search_index(val, A):
    N = len(A)
    for i in range(N):
        if val == A[i]:
            return i
    return -1

print(linear_search_index(3, [1,2,3,4,5]))
print(linear_search_index(5, [1,2,3,4,5]))
print(linear_search_index(100, [1,2,3,4,5]))
