Code error

I was writing code for fps game, while creating code for switching guns there was an error and i have no clue why it is not working.
My Code

using UnityEngine;

public class WeaponSwitching : MonoBehaviour
{

public int selectedWeapon = 0;

// Start is called before the first frame update
void Start()
{
    SelectWeapon();
}

// Update is called once per frame
void Update()
{
    int previousSelectedWeapon = selectedWeapon;

    if (Input.GetAxis("Mouse ScrollWheel") > 0f)
    {
        if (selectedWeapon >= transform.childCount - 1)
            selectedWeapon = 0;
        else
            selectedWeapon +;
    }
    if (Input.GetAxis("Mouse ScrollWheel") < 0f)
    {
        if (selectedWeapon <= 0)
            selectedWeapon = transform.childCount - 1;
        else
            selectedWeapon -;
    }
    if (Input.GetKeyDown(KeyCode.Alpha1))
    {
        selectedWeapon = 0;
    }
    if (Input.GetKeyDown(KeyCode.Alpha2) && transform.childCount >= 2)
    {
        selectedWeapon = 1;
    }
    if (Input.GetKeyDown(KeyCode.Alpha2) && transform.childCount >= 3)
    {
        selectedWeapon = 2;
    }

    if (previousSelectedWeapon != selectedWeapon)
    {
        SelectWeapon();
    }
}

void SelectWeapon()
{
    int i = 0;
    foreach (Transform weapon in transform)
    {
        if (i == selectedWeapon)
            weapon.gameObject.SetActive(true);
        else
            weapon.gameObject.SetActive(false);
        i +;
    }
}

}
the error is error CS1525: Invalid expression term ‘;’
this error is for
i +;
selectedWeapon -;
selectedWeapon +;

You have to write double + or - for increments and decrements.

i +; //error
i++; //adds 1 to i

same for the other variables.