How to get max key that contains partial value in dictionary in unity

This is for Unity (C#)

I have a this Dictionary :

public Dictionary<int, string> lvl_dict = new Dictionary<int, string>()
		{
			//Dict tuplas index (Unity), moduleStepCode (API)
			{1,"R1_G1_A1_M1_A"},
			{2,"R1_G1_A1_M1_P"},
			{3,"R1_G1_A1_M1_E"},
			{4,"R1_G1_A1_M2_A"},
			{5,"R1_G1_A1_M2_P"},
			{6,"R1_G1_A1_M2_E"},
			{7,"R1_G1_A1_M3_A"},
			{8,"R1_G1_A1_M3_P"},

I need to retrieve the maximum Key given a partial Value.

Example, If I input "R1_G1_A1_M2", I need to retrieve Key = 6.

I've seen answers that retrieve the Max key given a Value, but not a partial value, which should create a subset of the dictionary, and then retrieve the max key, I guess? But I don't know how to do that.

I tried using LINQ

var maxkey = lvl_dict.Where(item => item.Value.Contains("R1_G1_A1_M2").max() );   
Debug.Log("max key is : " + maxkey);

But clearly i don't know what i am doing.

There is two way to get max of keys:

1- using Max function

var max = lvl_dict.Where(x => x.Value.Contains("G1_A1_M2")).Select(x => x.Key).Max();

2- using MaxBy function

var max = lvl_dict.Where(x => x.Value.Contains("G1_A1_M2")).MaxBy(x => x.Key).Key;