Balázs received a pocket calculator for his birthday. He immediately set to work, adding whole numbers in sequence starting from 1. When the display showed 3000, he triumphantly looked at his brother, who had been watching his little brother's efforts from the start. At this point, his brother pointed out to Balázs that he had forgotten a number. Which one was it?
Let $n$ be the last term in the sum of 3000, and $k$ the forgotten number. Clearly $1 \leq k1$ ):

$$
\begin{array}{ll} 
& n \geq \frac{-1+\sqrt{24009}}{2} \approx 76.97 \\
\text { or } \quad & n<\frac{1+\sqrt{24001}}{2} \approx 77.96
\end{array}
$$

Since $n$ is an integer, only $n=77$ satisfies both inequalities. Therefore, the forgotten number $k=\frac{77 \cdot 78}{2}-3000=3$.