Code:

int mot1=9;
int mot2=6;
int mot3=5;
int mot4=3;
    
int left=13;
int right=12;
    
int Left=0;
int Right=0;
void LEFT (void);
void RIGHT (void);
void STOP (void);

void setup(){
    pinMode(mot1,OUTPUT);
    pinMode(mot2,OUTPUT);
    pinMode(mot3,OUTPUT);
    pinMode(mot4,OUTPUT);
    
    pinMode(left,INPUT);
    pinMode(right,INPUT);
    
    digitalWrite(left,HIGH);
    digitalWrite(right,HIGH);
      
      
}
    
void loop() 
{
     
    analogWrite(mot1,255);
    analogWrite(mot2,0);
    analogWrite(mot3,255);
    analogWrite(mot4,0);
    
    while(1)
    {
      Left=digitalRead(left);
      Right=digitalRead(right);
      
      if((Left==0 && Right==1)==1)
      LEFT();
      else if((Right==0 && Left==1)==1)
      RIGHT();
    }
}
    
void LEFT (void){
    analogWrite(mot3,0);
    analogWrite(mot4,30);
       
       
    while(Left==0)
    {
    Left=digitalRead(left);
    Right=digitalRead(right);
    if(Right==0)
    {
        int lprev=Left;
        int rprev=Right;
        STOP();
        while(((lprev==Left)&&(rprev==Right))==1)
        {
            Left=digitalRead(left);
            Right=digitalRead(right);
          }
        }
        analogWrite(mot1,255);
        analogWrite(mot2,0); 
       }
       analogWrite(mot3,255);
       analogWrite(mot4,0);
    }
    
void RIGHT (void)
{
    analogWrite(mot1,0);
    analogWrite(mot2,30);
    
    while(Right==0)
    {
        Left=digitalRead(left);
        Right=digitalRead(right);
        if(Left==0)
        {
          int lprev=Left;
          int rprev=Right;
         STOP();
          while(((lprev==Left)&&(rprev==Right))==1)
          {
             Left=digitalRead(left);
             Right=digitalRead(right);
          }
        }
        analogWrite(mot3,255);
        analogWrite(mot4,0);
        }
       analogWrite(mot1,255);
       analogWrite(mot2,0);
    }
void STOP (void)
{
    analogWrite(mot1,0);
    analogWrite(mot2,0);
    analogWrite(mot3,0);
    analogWrite(mot4,0);
      
}