Simple Code:
Vector3 currentCameraEulers;
float rotationDPI = 7f; // I didn't remeber how to call better this thing
void Update(){
float cameraX = Input.GetAxisRaw("Mouse X") * rotationDPI;
if(cameraX != 0f){
currentCameraEulers = new Vector3(0, cameraX, 0);
transform.eulerAngles += currentCameraEulers;
}
}