write a go solution for Description:
You're given an undirected graph with n nodes and m edges. Nodes are numbered from 1 to n.

The graph is considered harmonious if and only if the following property holds:

- For every triple of integers (l,m,r) such that 1<=l<m<r<=n, if there exists a path going from node l to node r, then there exists a path going from node l to node m.

In other words, in a harmonious graph, if from a node l we can reach a node r through edges (l<r), then we should able to reach nodes (l+1),(l+2),ldots,(r-1) too.

What is the minimum number of edges we need to add to make the graph harmonious?

Input Format:
The first line contains two integers n and m (3<=n<=200000 and 1<=m<=200000).

The i-th of the next m lines contains two integers u_i and v_i (1<=u_i,v_i<=n, u_ineqv_i), that mean that there's an edge between nodes u and v.

It is guaranteed that the given graph is simple (there is no self-loop, and there is at most one edge between every pair of nodes).

Output Format:
Print the minimum number of edges we have to add to the graph to make it harmonious.

Note:
In the first example, the given graph is not harmonious (for instance, 1<6<7, node 1 can reach node 7 through the path 1arrow2arrow7, but node 1 can't reach node 6). However adding the edge (2,4) is sufficient to make it harmonious.

In the second example, the given graph is already harmonious.. Output only the code with no comments, explanation, or additional text.