/**
	author:  shobonvip
	created: 2025.04.06 16:19:47
**/

#include<bits/stdc++.h>
using namespace std;

//* ATCODER
#include<atcoder/all>
using namespace atcoder;
typedef modint998244353 mint;
//*/

/* BOOST MULTIPRECISION
#include<boost/multiprecision/cpp_int.hpp>
using namespace boost::multiprecision;
//*/

typedef long long ll;

#define rep(i, s, n) for (int i = (int)(s); i < (int)(n); i++)
#define rrep(i, s, n) for (int i = (int)(n)-1; i >= (int)(s); i--)
#define all(v) v.begin(), v.end()

template <typename T> bool chmin(T &a, const T &b) {
	if (a <= b) return false;
	a = b;
	return true;
}

template <typename T> bool chmax(T &a, const T &b) {
	if (a >= b) return false;
	a = b;
	return true;
}

template <typename T> T max(vector<T> &a){
	assert(!a.empty());
	T ret = a[0];
	for (int i=0; i<(int)a.size(); i++) chmax(ret, a[i]);
	return ret;
}

template <typename T> T min(vector<T> &a){
	assert(!a.empty());
	T ret = a[0];
	for (int i=0; i<(int)a.size(); i++) chmin(ret, a[i]);
	return ret;
}

template <typename T> T sum(vector<T> &a){
	T ret = 0;
	for (int i=0; i<(int)a.size(); i++) ret += a[i];
	return ret;
}

int main(){
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	
	int n,m;
	cin>>n>>m;
	vector<int>s(m);
	rep(i,0,m){
		cin>>s[i];
		s[i]--;
	}
	vector<int>p(n);
	vector<int> pinv(n);
	iota(all(p),0);
	iota(all(pinv),0);
	vector<pair<int,int>>que(m);
	rep(i,0,m){
		que[i]=pair(p[s[i]],p[s[i]+1]);
		swap(p[s[i]],p[s[i]+1]);
		pinv[p[s[i]]]=s[i];
		pinv[p[s[i]+1]]=s[i]+1;
	}
	iota(all(p),0);
	iota(all(pinv),0);
	set<pair<int,int>>ans;
	rrep(i,0,m){
		ans.insert(pair(que[i].first,que[i].second));
		ans.insert(pair(que[i].second,que[i].first));
		swap(p[s[i]],p[s[i]+1]);
		pinv[p[s[i]]]=s[i];
		pinv[p[s[i]+1]]=s[i]+1;
	}
	cout<<(int)ans.size()/2<<endl;

}