#include <bits/stdc++.h>
using namespace std;
int main(void){
	int n,m;
	cin>>n>>m;
	vector<int> s(m);
	for (auto &e:s)cin>>e;
	vector<int> ord(n);
	iota(ord.begin(), ord.end(), 0);
	set<pair<int,int>> st;
	for (auto x : s){
		st.emplace(ord[x-1], ord[x]);
		st.emplace(ord[x], ord[x-1]);
		swap(ord[x-1], ord[x]);
	}
	cout<<st.size()/2<<endl;
}