Given a graph of $n$ vertices and $m$ edges. Determine whether or not the given graph is a tree.
### Input
- The first line contains 2 integers $n, m$.
- The next $n$ lines, each line contains 2 integers $u, v$, there is an edge between $u$ and $v$.
### Output
- Print `YES` if the graph is a tree, `NO` otherwise.
### Constraints
- $1 \le n, m \le 10^5$.
- $1 \le u, v \le n$.
### Example
Input:
```
6 5
1 2
2 3
3 4
4 5
5 6
```
Output:
```
YES
```
