Tachomter Completely Working Code

#include <LiquidCrystal.h>
LiquidCrystal lcd(50, 51, 49, 48, 47, 46);

int rpmcount = 0;
int rpm = 0;
unsigned long lastmillis = 0;
void setup(){
 Serial.begin(9600);
 attachInterrupt(2, rpm_fan, FALLING);
 lcd.begin(16,2);
 lcd.print("Current RPM:");
}
void loop(){
 if (millis() - lastmillis == 1000){ //Uptade every one second, this will be equal to reading frecuency (Hz).
 detachInterrupt(0);//Disable interrupt when calculating
 rpm = rpmcount*30; // Convert frecuency to RPM, note: this works for one interruption per full rotation. For two interrups per full rotation use rpmcount * 30.
 lcd.setCursor(0, 1);
  lcd.print("      ");
  lcd.setCursor(0, 1);
  lcd.print(rpm);
 Serial.print("RPM =\t"); //print the word "RPM" and tab.
 Serial.print(rpm); // print the rpm value.
 Serial.print("\t Hz=\t"); //print the word "Hz".
 Serial.println(rpmcount); //print revolutions per second or Hz. And print new line or enter.
 rpmcount = 0; // Restart the RPM counter
 lastmillis = millis(); // Uptade lasmillis
 attachInterrupt(0, rpm_fan, FALLING); //enable interrupt
  }
}
 // this code will be executed every time the interrupt 0 (pin2) gets low.
void rpm_fan(){
  rpmcount++;
}