#include <bits/stdc++.h>
using namespace std;
const int inf = 2000000000;
static inline int Rint() {
  struct X {
    int dig[256];
    X() {
      for (int i = '0'; i <= '9'; ++i) dig[i] = 1;
      dig['-'] = 1;
    }
  };
  static X fuck;
  int s = 1, v = 0, c;
  for (; !fuck.dig[c = getchar()];)
    ;
  if (c == '-')
    s = 0;
  else if (fuck.dig[c])
    v = c ^ 48;
  for (; fuck.dig[c = getchar()]; v = v * 10 + (c ^ 48))
    ;
  return s ? v : -v;
}
template <typename T>
static inline void cmax(T& a, const T& b) {
  if (b > a) a = b;
}
template <typename T>
static inline void cmin(T& a, const T& b) {
  if (b < a) a = b;
}
struct Pt {
  int x, y;
};
int cmp_x_min(const Pt& a, const Pt& b) {
  if (a.x != b.x) return a.x < b.x;
  return a.y < b.y;
}
Pt data[100005];
int main() {
  int n = Rint(), m = Rint();
  for (int i = 0; i < m; ++i) data[i].x = Rint(), data[i].y = Rint();
  sort(data, data + m, cmp_x_min);
  vector<int> last;
  last.push_back(1);
  last.push_back(1);
  int row = 0;
  for (int now = 0; now < m;) {
    int j = now + 1;
    while (j < m && data[j].x == data[now].x) ++j;
    if (last.size() && data[now].x > row + 1) {
      vector<int> next;
      next.push_back(last.front()), next.push_back(n);
      last.swap(next);
      row = row + 1;
    }
    const int size = ((int)(last).size());
    vector<int> next;
    for (int k = now, pos = 0; k <= j; ++k) {
      int r = k == j ? n : data[k].y - 1;
      int l = k == now ? 1 : data[k - 1].y + 1;
      if (l > r) continue;
      while (pos < size && last[pos + 1] < l) pos += 2;
      if (pos < size) {
        if (last[pos] > r) continue;
        if (last[pos] <= l) {
          next.push_back(l);
          next.push_back(r);
        } else if (last[pos] <= r) {
          next.push_back(last[pos]);
          next.push_back(r);
        }
      }
    }
    last.swap(next);
    row = data[now].x;
    now = j;
  }
  if (last.size() && data[m - 1].x < n) {
    vector<int> next;
    next.push_back(last.front()), next.push_back(n);
    last.swap(next);
    row = row + 1;
  }
  if (last.size()) {
    const int size = ((int)(last).size());
    if (last[size - 2] <= n && last[size - 1] >= n) {
      printf("%d\n", 2 * n - 2);
      return 0;
    }
  }
  puts("-1");
  return 0;
}
