Problem with error 'void value not ignored as it ought to be' in Arduino IDE 1.6.7

error_message.txt

/home/viciu/akd-sketchbook/not_ignored_void/tab2.ino: In function 'void setup()':
tab2:3: error: void value not ignored as it ought to be
   Serial.println("2+2=4");
                     ^
exit status 1
void value not ignored as it ought to be

not_ignored_void.ino

void setup() {
  Serial.begin(9600);
  int val = add_two();
  Serial.println(val);
  
}

void loop() {
}

tab2.ino

void add_two() {
  Serial.println("2+2=4");
}