how to make arduino nano pro low power

hi

i want make arduino code set low power. consume 20ma. want reduce it. how it.

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const int buttonpin = 2;
const int ledpin =  13;

byte ledflag = 0;
byte enableflag = 0;
unsigned long ledtime;
unsigned long flashtime;
const unsigned long flashrate = 100ul;

int buttonstate = 0;



void setup()
{
 
  pinmode(ledpin, output);

  pinmode(buttonpin, input_pullup);
}

void loop()
{
  buttonstate = digitalread(buttonpin);

  if (buttonstate == high && ledflag == 0)
  {
    digitalwrite(ledpin, high);
    ledflag = 1;
    enableflag = 1;
    ledtime = millis();
    flashtime = millis();
  }

  if (enableflag == 1 && millis() - ledtime <= 4500ul)
  {
    if (millis() - flashtime >= flashrate)
    {
      flashtime = millis();
      digitalwrite(ledpin, !digitalread(ledpin));
    }
  }
  else
  {
    enableflag = 0;
    digitalwrite(ledpin, low);
  }

  if (buttonstate == low)
  {
    ledflag = 0;
    enableflag = 0;
    digitalwrite(ledpin, low);
  }

}

have considered sleep mode?
type "sleep" in search box @ top of page.
good luck.

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