I was given android apk file [chargetracker.apk](https://github.com/mar232320/ctf-writeups/raw/main/chargetracker.apk)
I looked in internet for online apk decompilers decompiled the file and looked for the java source files The most interesting one was `MainActivity.java`
```package com.bsdiessf.chargetracker;
import android.content.BroadcastReceiver;import android.content.Context;import android.content.Intent;import android.content.IntentFilter;import android.os.BatteryManager;import android.os.Bundle;import android.util.Log;import android.widget.ImageView;import android.widget.TextView;import androidx.appcompat.app.AppCompatActivity;import androidx.appcompat.widget.Toolbar;
public class MainActivity extends AppCompatActivity {    private ImageView batteryImage;    private TextView batteryValue;    private BroadcastReceiver mBatteryReceiver = new BroadcastReceiver() {        /* class com.bsdiessf.chargetracker.MainActivity.AnonymousClass1 */
        public void onReceive(Context context, Intent intent) {            int intExtra = intent.getIntExtra("level", 0);            String valueOf = String.valueOf(intExtra);            Log.d("Battery update:", valueOf);            if (intExtra == 49) {                Log.d("Flag:", MainActivity.this.getResources().getString(R.string.part1) + MainActivity.this.part2 + "1tN0w}");            }            TextView textView = MainActivity.this.batteryValue;            textView.setText(valueOf + "%");            if (intExtra > 79) {                MainActivity.this.batteryImage.setImageResource(R.drawable.high);            } else if (intExtra <= 30 || intExtra >= 80) {                MainActivity.this.batteryImage.setImageResource(R.drawable.low);            } else {                MainActivity.this.batteryImage.setImageResource(R.drawable.med);            }        }    };    private String part2 = "R3charg3";
    /* access modifiers changed from: protected */    @Override // androidx.activity.ComponentActivity, androidx.core.app.ComponentActivity, androidx.appcompat.app.AppCompatActivity, androidx.fragment.app.FragmentActivity    public void onCreate(Bundle bundle) {        super.onCreate(bundle);        setContentView(R.layout.activity_main);        setSupportActionBar((Toolbar) findViewById(R.id.toolbar));        this.batteryImage = (ImageView) findViewById(R.id.batteryImageView);        this.batteryValue = (TextView) findViewById(R.id.percentValue);        int intProperty = ((BatteryManager) getSystemService("batterymanager")).getIntProperty(4);        TextView textView = this.batteryValue;        textView.setText(String.valueOf(intProperty) + "%");        registerReceiver(this.mBatteryReceiver, new IntentFilter("android.intent.action.BATTERY_CHANGED"));    }
    /* access modifiers changed from: protected */    @Override // androidx.appcompat.app.AppCompatActivity, androidx.fragment.app.FragmentActivity    public void onDestroy() {        super.onDestroy();        unregisterReceiver(this.mBatteryReceiver);    }}
```
I could see `1tN0w}` as last part at the end and `R3charg3` in `private String part2` I assumed that the first part is `CTF{`
> CTF{R3charg31tN0w}