Solar tracker by LDR and Servo Motor

I am posting the code that I have used but here movement of the servo is not being possible by just by adding and subtracting (I have underlined and bolded the line) , please provide me any other ulternative or a different source code for the SOLAR TRACKER USING LDR AND SERVO MOTOR.

#include <Servo.h> //including the library of servo motor
Servo sg90; //initializing a variable for servo named sg90
int initial_position = 90; //Declaring the initial position at 90
int LDR1 = A0; //Pin at which LDR is connected
int LDR2 = A1; //Pin at which LDR is connected
int error = 5; //initializing variable for error
int servopin=9;
void setup()
{
sg90.attach(servopin); // attaches the servo on pin 9
pinMode(LDR1, INPUT); //Making the LDR pin as input
pinMode(LDR2, INPUT);
sg90.write(initial_position); //Move servo at 90 degree
delay(2000); // giving a delay of 2 seconds
}

void loop()
{
int R1 = analogRead(LDR1); // reading value from LDR 1
int R2 = analogRead(LDR2); // reading value from LDR 2
int diff1= R1 - R2; // Calculating the difference between the LDR’s
int diff2= R2 - R1;

if((diff1 <= error) || (diff2 <= error)) {
//if the difference is under the error then do nothing
} else {
if(R1 > R2)
{
initial_position = --initial_position; //Move the servo towards 0 degree
}
if(R1 < R2)
{
** initial_position = ++initial_position;** //Move the servo towards 180 degree
}
}
sg90.write(initial_position); // write the position to servo
delay(100);
}

  initial_position = --initial_position;  //Move the servo towards 0 degree
    }
    if(R1 < R2)
    {
      initial_position = ++initial_position;

The syntax of those decrement and increment statements is wrong.

The proper syntax is:

initial_position--;
// or
 initial_position++;

// or
initial_position =  initial_position - 1;
// or
 initial_position =  initial_position + 1;

//or
initial_position -=  1;
// or
 initial_position +=  1;

... or "++initial_position;" or "--initial_position;"