#include <bits/stdc++.h>
using namespace std;
const long long MOD = (1 ? 100000000000000007 : 233333333333333333);
const int mod = (1 ? 1000000007 : 998244353);
const long long INF = 0x7fffffffffffffff;
const int inf = 0x7fffffff;
const double eps = 1e-8;
const int N = 1e5 + 5;
clock_t start, finish;
void time_in() { start = clock(); }
void time_out() {
  finish = clock();
  double tim = (double)(finish - start) / CLOCKS_PER_SEC;
  printf("Running time is %lf\n", tim);
}
inline long long mul(long long a, long long b) {
  long long s = 0;
  while (b) {
    if (b & 1) s = (s + a) % mod;
    a = (a << 1) % mod;
    b >>= 1;
  }
  return s % mod;
}
inline long long pow_mul(long long a, long long b) {
  long long s = 1;
  while (b) {
    if (b & 1) s = mul(s, a);
    a = mul(a, a);
    b >>= 1;
  }
  return s;
}
inline long long poww(long long a, long long b) {
  long long s = 1;
  while (b) {
    if (b & 1) s = (s * a) % mod;
    a = (a * a) % mod;
    b >>= 1;
  }
  return s % mod;
}
inline int read() {
  char c = getchar();
  int sgn = 1, ret = 0;
  while (c != '-' && (c < '0' || c > '9')) c = getchar();
  if (c == '-') sgn = -1, c = getchar();
  while (c >= '0' && c <= '9') ret = ret * 10 + c - 48, c = getchar();
  return ret * sgn;
}
void write(int x) {
  if (x > 9) write(x / 10);
  putchar(x % 10 + '0');
}
inline long long phi(long long x) {
  long long ans = x;
  for (long long i = 2; i * i <= x; i++)
    if (x % i == 0) {
      ans -= ans / i;
      while (x % i == 0) x /= i;
    }
  if (x > 1) ans -= ans / x;
  return ans;
}
int st[8][2] = {1, 0, 0, 1, -1, 0, 0, -1, 1, 1, -1, -1, 1, -1, -1, 1};
int t, cas;
int n, m;
int a[N];
int main() {
  while (~scanf("%d%d", &n, &m)) {
    memset(a, 0, sizeof(a));
    int mi = N, ma = 0;
    if (m == 0) {
      printf("%d\n", n - 1);
      continue;
    }
    for (int i = 0; i < m; i++) {
      int x, y;
      scanf("%d%d", &x, &y);
      if (a[x] == 0 && a[y] == 0) {
        if (x < y) swap(x, y);
        mi = min(mi, x);
        a[x] = 1;
        ma = max(ma, y);
        a[y] = 2;
      } else if (a[x] == 0) {
        if (a[y] == 1) {
          a[x] = 2;
          ma = max(ma, x);
        } else {
          a[x] = 1;
          mi = min(mi, x);
        }
      } else if (a[y] == 0) {
        if (a[x] == 1) {
          a[y] = 2;
          ma = max(ma, y);
        } else {
          a[y] = 1;
          mi = min(mi, y);
        }
      } else {
        if (a[x] == a[y]) {
          ma = N, mi = 0;
        }
      }
    }
    printf("%d\n", max(0, mi - ma));
  }
}
