#define m11 11 // rear motor
#define m12 12
#define m21 10 // front motor
#define m22 9

char str[2], i;

void forward()
{
digitalWrite(m11, LOW);
digitalWrite(m12, LOW);
digitalWrite(m21, HIGH);
digitalWrite(m22, LOW);
}

void backward()
{
digitalWrite(m11, LOW);
digitalWrite(m12, LOW);
digitalWrite(m21, LOW);
digitalWrite(m22, HIGH);
}

void left()
{
digitalWrite(m11, HIGH);
digitalWrite(m12, LOW);
delay(100);
digitalWrite(m21, HIGH);
digitalWrite(m22, LOW);
}

void right()
{
digitalWrite(m11, LOW);
digitalWrite(m12, HIGH);
delay(100);
digitalWrite(m21, HIGH);
digitalWrite(m22, LOW);
}

void Stop()
{
digitalWrite(m11, LOW);
digitalWrite(m12, LOW);
digitalWrite(m21, LOW);
digitalWrite(m22, LOW);
}

void setup()
{
Serial.begin(9600);
pinMode(m11, OUTPUT);
pinMode(m12, OUTPUT);
pinMode(m21, OUTPUT);
pinMode(m22, OUTPUT);
}

void loop()
{
while (Serial.available())
{
char ch = Serial.read();
str[i++] = ch;

if (str[i - 1] == 'F')
{
Serial.println("Forward");
forward();
i = 0;
}

else if (str[i - 1] == 'L')
{
Serial.println("Left");
right();
i = 0;
}

else if (str[i - 1] == 'R')
{
Serial.println("Right");
left();
i = 0;
}

else if (str[i - 1] == 'B')
{
Serial.println("Backward");
backward();
i = 0;
}

else if (str[i - 1] == 'S')
{
Serial.println("Stop");
Stop();
i = 0;
}
delay(100);
}
}