#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("sse2")
using namespace std;
template <typename Arg1>
void __f(const char* name, Arg1&& arg1) {
  std::cerr << name << " : " << arg1 << '\n';
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args) {
  const char* comma = strchr(names + 1, ',');
  std::cerr.write(names, comma - names) << " : " << arg1 << " | ";
  __f(comma + 1, args...);
}
long long max(long long a, long long b) { return (a > b) ? a : b; }
long long min(long long a, long long b) { return (a < b) ? a : b; }
void amax(long long& a, long long b) { a = max(a, b) % 1000000007; }
void amin(long long& a, long long b) { a = min(a, b) % 1000000007; }
void inc(long long& a, long long b) { a = (a + b) % 1000000007; }
void dec(long long& a, long long b) { a = (a - b + 1000000007) % 1000000007; }
long long solve() {
  long long n, p;
  cin >> n >> p;
  vector<long long> in(n + 10, 1e9), out(n + 10, 0);
  for (long long i = 0; i < p; i++) {
    long long x;
    cin >> x;
    amin(in[x], i + 1);
    amax(out[x], i + 1);
  }
  long long ans = 0;
  for (long long i = 1; i < n + 1; i++) {
    if (out[i] == 0) {
      ans++;
    }
    long long cur = out[i];
    if (in[i] > out[i + 1] && i + 1 <= n) ans++;
    if (in[i] > out[i - 1] && i > 1) ans++;
  }
  cout << ans;
  return 0;
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long t = 1;
  while (t--) {
    solve();
  }
}
