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

int main(){
    int N, M; cin >> N >> M;
    vector<int> S(M);
    for (int i=0; i<M; i++){
        cin >> S[i];
        S[i]--;
    }

    vector<int> res(N);
    for (int i=0; i<N; i++){
        res[i] = i;
    }

    set<pair<int, int>> ans;
    for (int i=0; i<M; i++){
        int next_s1 = res[S[i]], next_s2 = res[S[i] + 1];
        res[S[i]] = next_s2, res[S[i] + 1] = next_s1;
        ans.insert({next_s1, next_s2});
        ans.insert({next_s2, next_s1});
    }

    cout << (int)(ans.size() / 2) << endl;
}