Number of Vertex (vcount) is one more than the actual

I am learning about igraph and get stuck on vcount.

The edgelist in triangle.txt:

1 2

2 3

3 1

```
> g<- read_graph("triangle.txt", format="edgelist")
> g
IGRAPH 35ad1ec D--- 4 3 --
+ edges from 35ad1ec:
[1] 2->3 3->4 4->2
> vcount(g)
[1] 4
```

It is clear that the number of vertex is 3 from the edgelist, but why R give the number of vertex is 4.

I did experiment with with two other edgelists, with number of vertex 4 and 34, they gives 5 and 35.

Thank you.