#include <bits/stdc++.h>
const int mod = 1e9 + 7;
const int maxn = 1e5 + 5;
const double eps = 1e-6;
using namespace std;
bool eq(const double &a, const double &b) { return fabs(a - b) < eps; }
bool ls(const double &a, const double &b) { return a + eps < b; }
bool le(const double &a, const double &b) { return eq(a, b) || ls(a, b); }
long long gcd(long long a, long long b) { return a == 0 ? b : gcd(b % a, a); };
long long lcm(long long a, long long b) { return a / gcd(a, b) * b; }
long long kpow(long long a, long long b) {
  long long res = 1;
  a %= mod;
  if (b < 0) return 1;
  for (; b; b >>= 1) {
    if (b & 1) res = res * a % mod;
    a = a * a % mod;
  }
  return res;
}
long long read() {
  long long x = 0, f = 1;
  char ch = getchar();
  while (ch < '0' || ch > '9') {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    x = x * 10 + ch - '0';
    ch = getchar();
  }
  return x * f;
}
vector<pair<int, int> > can, tmp;
pair<int, int> p[maxn];
int main() {
  int n, m, x, y;
  scanf("%d%d", &n, &m);
  for (int i = 1; i < (m + 1); ++i)
    scanf("%d%d", &x, &y), p[i] = make_pair(x, y);
  sort(p + 1, p + m + 1);
  int t = 1, tt, nowr = 0;
  int be, ed;
  can.push_back(make_pair(1, 1));
  while (t <= m) {
    if (can.size() && p[t].first != nowr + 1) {
      be = can[0].first;
      can.clear();
      can.push_back(make_pair(be, n));
    }
    int id = 0;
    tt = t, be = 1, tmp.clear();
    while (p[t].first == p[tt].first) tt++;
    for (int i = t; i < (tt + 1); ++i) {
      ed = (i == tt) ? n : p[i].second - 1;
      for (; id < can.size() && can[id].second < be; ++id)
        ;
      if (id < can.size() && max(be, can[id].first) <= ed) {
        tmp.push_back(make_pair(max(be, can[id].first), ed));
        int l = max(be, can[id].first), r = ed;
      }
      be = ed + 2;
    }
    can = tmp;
    nowr = p[t].first;
    t = tt;
  }
  printf("%d",
         (can.size() && (nowr < n || can[(int)can.size() - 1].second == n))
             ? 2 * n - 2
             : -1);
  return 0;
}
