#include <bits/stdc++.h>
using namespace std;
#define rep(i, s, n) for (int i = (s); i < (int)(n); i++)
#include <atcoder/all>
using namespace atcoder;
using mint1 = modint1000000007;
using mint2 = modint998244353;
using ll = long long;
using ull = unsigned long long;
using ld = long double;
ld pi=3.141592653589793;
const int inf=2e9;
const ll linf=2e18;
const ld eps=1e-14;
#define Yes cout << "Yes" << endl
#define No cout << "No" << endl

ll dx[4]={1,0,-1,0};
ll dy[4]={0,1,0,-1};

struct edge{
    ll to;
    ll leng;
};

int main() {
    //////////////////
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    //////////////////

    ll n,m;
    cin >> n >> m;
    vector<ll> s(m);
    for(ll i=0; i<m; i++) {
        cin >> s[i];
    }
    vector<ll> a(n);
    iota(a.begin(), a.end(), 0);
    set<pair<ll,ll>> st;
    for(ll i=0; i<m; i++) {
        st.insert({a[s[i]-1],a[s[i]]});
        st.insert({a[s[i]],a[s[i]-1]});
        swap(a[s[i]-1],a[s[i]]);
    }
    cout << st.size()/2 << endl;

}