#include <bits/stdc++.h>

using namespace std;

using ll = long long;
using ld = long double;

const int N = 1003;

bool a[N];

int main() {
    ios::sync_with_stdio(false); cout.tie(nullptr); cin.tie(nullptr);
    int s, n;
    cin >> s >> n;
    for (int i = 0; i < n; ++i) {
        int x; cin >> x; --x;
        a[x] = true;
    }
    int cnt = 0;
    for (int i = 0; i < s; ++i) {
        if (a[i]) {
            ++i; continue;
        }
        if ((0 < i or not a[s - 1]) and (i + 1 < s and not a[i + 1])) {
            ++i; ++cnt;
        }
    }
    cout << cnt << '\n';
    return 0;
}