I don't know how to find Average Seasonal Index and Deseasonalized columns

Month	Demand	Yearly average	Seasonal Index	Average SI	Deseasonalized	Time period
1/1/19	2422		0.8226	1.0000	0.72	1
2/1/19	2564		0.8708	1.0051	0.76	2
3/1/19	3080		1.0460	1.0090	0.91	3
4/1/19	3004		1.0202	1.0079	0.89	4
5/1/19	2595		0.8813	1.0075	0.77	5
6/1/19	3654		1.2410	1.0116	1.09	6
7/1/19	2725		0.9255	1.0039	0.81	7
8/1/19	2383		0.8093	1.0066	0.71	8
9/1/19	2432		0.8260	1.0137	0.72	9
10/1/19	2744		0.9319	1.0206	0.82	10
11/1/19	3600		1.2227	1.0241	1.07	11
12/1/19	4130	2944.42	1.4027	1.0161	1.23	12
1/1/20	2634		0.7794	1.0000	0.78	13
2/1/20	2508		0.7421	1.0096	0.75	14
3/1/20	2653		0.7850	1.0218	0.79	15
4/1/20	3270		0.9676	1.0330	0.97	16
5/1/20	3770		1.1155	1.0363	1.12	17
6/1/20	3588		1.0617	1.0321	1.07	18
7/1/20	2658		0.7865	1.0305	0.79	19
8/1/20	3065		0.9069	1.0448	0.91	20
9/1/20	3095		0.9158	1.0535	0.92	21
10/1/20	3634		1.0753	1.0626	1.08	22
11/1/20	4350		1.2871	1.0617	1.29	23
12/1/20	5330	3379.583333	1.5771	1.0444	1.58	24
1/1/21	2810		0.7444	1.0000	0.83	25
2/1/21	2999		0.7945	1.0232	0.89	26
3/1/21	3051		0.8083	1.0461	0.91	27
4/1/21	3653		0.9677	1.0725	1.09	28
5/1/21	3709		0.9826	1.0856	1.10	29
6/1/21	3930		1.0411	1.1004	1.17	30
7/1/21	2949		0.7812	1.1102	0.88	31
8/1/21	3348		0.8869	1.1760	0.99	32
9/1/21	3400		0.9007	1.2483	1.01	33
10/1/21	4361		1.1553	1.3642	1.30	34
11/1/21	5079		1.3455	1.4686	1.51	35
12/1/21	6008	3774.75	1.5916	1.5916	1.78	36
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