Sum of Natural number

Take any 2 circles among the given blue circles.
Draw 2 lines inclined at $60 ^{\circ}$ to each other .
For every such pair of points we get a corresponding yellow point.
If the total number of blue circles is $n$,the number of ways to select 2 circles = $\binom{n}{2}$.
Count the total number of yellow circles.
Now I guess it is easy to see that:
$$1+2+\cdots + (n-1) = \binom{n}{2}$$