#include <bits/stdc++.h>
using namespace std;
long long add(long long x, long long y) {
  x += y;
  while (x >= 998244353) x -= 998244353;
  while (x < 0) x += 998244353;
  return x;
}
long long mul(long long x, long long y) { return (x * 1ll * y) % 998244353; }
long long binpow(long long x, long long y) {
  long long z = 1;
  while (y > 0) {
    if (y % 2 == 1) z = mul(z, x);
    x = mul(x, x);
    y /= 2;
  }
  return z;
}
long long inv(long long x) { return binpow(x, 998244353 - 2); }
long long divide(long long x, long long y) { return mul(x, inv(y)); }
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long n, k;
  cin >> n >> k;
  vector<long long> a(k);
  for (long long i = 0; i < (long long)k; ++i) cin >> a[i];
  vector<long long> last(n + 1, -1), fst(n + 1, k);
  for (long long i = 1; i <= k; i++) {
    last[a[i - 1]] = i;
  }
  for (long long i = k; i >= 1; i--) {
    fst[a[i - 1]] = i;
  }
  long long ans = 0;
  for (long long i = 1; i <= n; i++) {
    for (long long j = max(1ll, i - 1); j <= min(i + 1, n); j++) {
      if (j == i) {
        if (last[i] == -1) ans += 1;
      } else
        ans += (last[i] <= fst[j]);
    }
  }
  cout << ans << endl;
  return 0;
}
