351. Find the general form of natural numbers that can be represented as the sum of consecutive natural numbers, starting from one.
Instruction. The task is to determine the expression for the sum of $n$ terms of an arithmetic progression, where the first term and the difference are equal to one, i.e.

$$
S_{n}=\frac{n(n+1)}{2}
$$

Thus, the required numbers will be: $1,3,6,10,15,21 \ldots$