# M0155

This warning indicates that the type of a subtraction operation had to be deduced from its operands and was inferred to be `Nat`.
That implies that it traps when the result is negative, which may be unintentional.

Offending code examples:

```motoko
func f(n : Nat) {
  if (n < 10) { return };
  let m = 2 * (n - 1);
};

func g(n : Nat) {
  if (n - 1 < 10) { return };
};
```

If the subtraction was indeed intended to have a `Nat` result, you can let the compiler know by annotating the intended type explicitly:

```motoko
func f(n : Nat) {
  let m : Nat = 2 * (n - 1);
};
```

If the intended type was `Int`, however, you can either annotate it as such:

```
func f(n : Nat) {
  let m : Int = 2 * (n - 1);
};
```

Or you can insert a sign operator `+`, which also forces the expression to be of type `Int`:

```
func f(n : Nat) {
  let m = 2 * (+n - 1);
};
```

This latter possibility is particularly convenient in the case of comparisons, because it is always okay to perform them at type `Int`:

```
func g(n : Nat) {
  if (+n - 1 < 10) { return };
};
```