I see many people forgetting about multiplication symbol.

Two common mistakes.

change

```
a *= -1;
```

to

```
a = -a;
```

change

```
res = res * symbol;
```

to

```
res = (symbol == -1)? res = -res: res;
```

or (make symbol into boolean)

`res = (neg)? res = -res: res;`