/**
 * @author : cyb1010
 * @date   : 2023-07-18 10:11:09
 * @file   : trolcon.cpp
 */
#include <bits/stdc++.h>
using namespace std;
#define fo(s) freopen(s ".in", "r", stdin), freopen(s ".out", "w", stdout)
#define fi first
#define se second
typedef double db;
typedef long double ldb;
typedef long long ll;
typedef unsigned long long ull;
int n, f[100010][5], lst[100010], cont[100010], ans[5];
struct seg {
    int l, r;
    seg() {}
    seg(int _l, int _r) : l(_l), r(_r) {}
} a[100010];
set<pair<int, int>> s;
bool cmp(seg x, seg y) { return x.r < y.r; }
struct point {
    int l, r, t;
} t[3000010];
int rt, tot, inf = 1000000000;
int mx(int x, int y) { return a[x].l > a[y].l ? y : x; }
void add(int &p, int l, int r, int k) {
    if (!p)
        p = ++tot, t[p].t = k;
    t[p].t = mx(t[p].t, k);
    if (l < r) {
        int mid = l + r >> 1;
        a[k].r <= mid ? add(t[p].l, l, mid, k) : add(t[p].r, mid + 1, r, k);
    }
}
int qry(int p, int l, int r, int ll) {
    if (r < ll || !p)
        return 0;
    if (l >= ll)
        return t[p].t;
    int mid = l + r >> 1;
    return mx(qry(t[p].l, l, mid, ll), qry(t[p].r, mid + 1, r, ll));
}
int main() {
    // fo("trolcon");
    scanf("%d", &n), a[0].l = inf;
    for (int i = 1; i <= n; i++) scanf("%d%d", &a[i].l, &a[i].r);
    sort(a + 1, a + 1 + n, cmp);
    for (int i = 1; i <= n; i++) {
        auto x = s.lower_bound({a[i].l, 0});
        if (x != s.begin())
            lst[i] = (--x)->second;
        cont[i] = qry(rt, 1, inf, a[i].l), add(rt, 1, inf, i);
        s.emplace(a[i].r, i), !cont[i] ? cont[i] = i : 1;
    }
    for (int s = 0; s <= 3; s++) {
        for (int i = 1; i <= n; i++) {
            f[i][s] = f[lst[i]][s] + 1;
            int p = cont[i];
            for (int j = 0; j <= s; j++)
                f[i][s] = min(f[i][s], f[lst[p]][s - j] + 1), p = cont[p];
            // printf("%d ", f[i][s]);
        }
        // putchar('\n');
        ans[s] = f[n][s];
        // for (int i = lst[n] + 1; i < n; i++) ans[s] = min(ans[s], f[i][s]);
        // printf("%d\n", ans[s]);
    }
    if (ans[1] < ans[0])
        printf("1\n");
    else if (ans[2] < ans[0])
        printf("2\n");
    else if (ans[3] < ans[0])
        printf("3\n");
    else
        printf("impossible\n");
    return 0;
}