how i -10 from thing in another code

i need to -10 from thing in another code like a that

using System.Collections;
using System.Collections.Generic;
using UnityEngine.SceneManagement;
using UnityEngine;

public class Buy_H : MonoBehaviour
{

// Use this for initialization
void Start()
{

}

// Update is called once per frame
void Update()
{

}
public void BackToGame()
{
    SceneManager.LoadScene("Main");
}

public void BUY_H()
{
    if (DataManegement.datamanegement.AllCrystals < 20)
    {
        Debug.Log("ﺔﻴﻓﺎﻛ ﺮﻫﺍﻮﺟ ﻚﻳﺪﻟ ﺲﻴﻟ");
    }
    if (DataManegement.datamanegement.AllCrystals > 20)
    {
        DataManegement.datamanegement.AllCrystals - 10;
    }

}

}

the another code *****************************************

using System.Collections;
using System.Collections.Generic;
using System;
using System.Runtime.Serialization.Formatters.Binary;
using System.IO;
using UnityEngine;

public class DataManegement : MonoBehaviour
{

public static DataManegement datamanegement;

//my variables
public int tokensHighScore;
public int TotalTokensCollcted;
public int AllCrystals;

void Awake()
{
    Environment.SetEnvironmentVariable("MONO_REFLECTION_SERIALIZER", "yes");
    datamanegement = this;
    DontDestroyOnLoad (gameObject);
}

public void SaveData()
{
    BinaryFormatter binFrom = new BinaryFormatter();
    FileStream file = File.Create(Application.persistentDataPath + "gameInfo.dat");
    gameData data = new gameData();
    data.tokensHighScore = tokensHighScore;
    data.TotalTokensCollcted = TotalTokensCollcted;
    data.AllCrystals = AllCrystals;
    binFrom.Serialize(file, data);
    file.Close();
}
public void LoadData()
{
    if (File.Exists(Application.persistentDataPath + "gameInfo.dat"))
    {
        BinaryFormatter binForm = new BinaryFormatter();
        FileStream file = File.Open(Application.persistentDataPath + "gameInfo.dat", FileMode.Open);
        gameData data = (gameData)binForm.Deserialize(file);
        file.Close();
        tokensHighScore = data.tokensHighScore;
        TotalTokensCollcted = data.TotalTokensCollcted;
        AllCrystals = data.AllCrystals;
    }

}

}
[Serializable]
class gameData
{
public int tokensHighScore;
public int TotalTokensCollcted;
public int AllCrystals;
}

Hi
I think you did wrong the subtraction.
Write this:
AllCrystals -= 10
Or:
AllCrystals = AllCrystals - 10