/*
* Author: Enucai
* Date: 2022-12-13 09:51:34
* LastEditTime: 2023-06-29 20:21:13
*/

#include <bits/stdc++.h>
#ifdef dbg
#define D(...) fprintf(stderr, __VA_ARGS__)
#define DD(...) D(#__VA_ARGS__ " = "), debug_helper::debug(__VA_ARGS__), D("\n")
#include "C:\Users\wsyear\Desktop\OI\templates\debug.hpp"
#else
#define D(...) ((void)0)
#define DD(...) ((void)0)
#endif
#define rep(i, j, k) for (int i = (j); i <= (k); ++i)
#define per(i, j, k) for (int i = (j); i >= (k); --i)
#define SZ(v) int((v).size())
#define ALL(v) (v).begin(),(v).end()
#define fi first
#define se second
#define gc getchar
#define pc putchar
using ll = long long;
using pii = std::pair<int, int>;
using pll = std::pair<ll, ll>;

using namespace std;

template <class T = int> T read() {
  T x = 0; bool f = 0; char ch = gc();
  while (!isdigit(ch)) f = ch == '-', ch = gc();
  while (isdigit(ch)) x = (x << 3) + (x << 1) + (ch ^ 48), ch = gc();
  return f ? -x: x;
}
template <class T> void write(T x) {
  if (x >= 0) { if (x > 9) write(x / 10); pc(x % 10 + 48); }
  else { pc('-'); if (x < -9) write(-x / 10); pc(48 - x % 10); }
}

const int N = 1000010;
const int M = 2000010;

struct SegMent {
  int l, r;
} a[N];

int n, tot, c[M], f[N][4][2], g[4], lst[N], con[N], t[M << 2], sm[N];

int mo(int op, int x, int y) {
  if (op) return min(x, y);
  else return max(x, y);
}

#define mid ((l + r) >> 1)

void upd(int c, int l, int r, int x, int v, int o) {
  if (l == r) return t[c] = mo(o, t[c], v), void();
  if (x <= mid) upd(c << 1, l, mid, x, v, o);
  else upd(c << 1 | 1, mid + 1, r, x, v, o);
  t[c] = mo(o, t[c << 1], t[c << 1 | 1]);
}

int qry(int c, int l, int r, int x, int y, int o) {
  if (l == x && r == y) return t[c];
  if (y <= mid) return qry(c << 1, l, mid, x, y, o);
  else if (x > mid) return qry(c << 1 | 1, mid + 1, r, x, y, o);
  else return mo(o, qry(c << 1, l, mid, x, mid, o), qry(c << 1 | 1, mid + 1, r, mid + 1, y, o));
}

#undef mid

int main() {
  n = read();
  rep (i, 1, n) a[i].l = read(), a[i].r = read();
  sort(a + 1, a + n + 1, [&](SegMent x, SegMent y) {
    return x.r == y.r ? x.l < y.l : x.r < y.r;
  });
  bool flag = 0;
  rep (i, 1, n - 1) if (a[i].r >= a[i + 1].l) flag = 1;
  if (!flag) return puts("impossible"), 0;
  rep (i, 1, n) c[2 * i - 1] = a[i].l, c[2 * i] = a[i].r;
  sort(c + 1, c + 2 * n + 1);
  tot = unique(c + 1, c + 2 * n + 1) - c - 1;
  rep (i, 1, n) {
    a[i].l = lower_bound(c + 1, c + tot + 1, a[i].l) - c;
    a[i].r = lower_bound(c + 1, c + tot + 1, a[i].r) - c;
  }
  rep (i, 1, tot << 2) t[i] = 0;
  rep (i, 1, n) {
    if (a[i].l != 1) lst[i] = qry(1, 1, tot, 1, a[i].l - 1, 0);
    upd(1, 1, tot, a[i].r, i, 0);
  }
  rep (i, 1, tot << 2) t[i] = n + 1;
  rep (i, 1, n) {
    con[i] = qry(1, 1, tot, a[i].l, a[i].r, 1);
    upd(1, 1, tot, a[i].r, i, 1);
  }
  rep (i, 0, n + 1) rep (j, 0, 3) f[i][j][0] = f[i][j][1] = 1E9;
  f[0][0][0] = 0;
  rep (i, 1, n) rep (j, 0, 3) {
    f[i][j][1] = min(f[lst[i]][j][1], f[lst[i]][j][0]) + 1;
    f[i][j][0] = f[con[i]][j][1];
    int pos = i;
    rep (k, 1, j) {
      pos = con[pos] != n + 1 ? con[pos] : pos;
      f[i][j][0] = min(f[i][j][0], f[pos][j - k][0]);
      f[i][j][1] = min(f[i][j][1], f[pos][j - k][1]);
    }
  }
  int res = min(f[n][0][0], f[n][0][1]);
  rep (i, 1, 3) if (min(f[n][i][0], f[n][i][1]) < res) return write(i), 0;
  puts("impossible");
}