#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define vt vector
#define pb push_back
#define pii pair<int, int>
#define sz(dq) (int)dq.size()
#define forr(i, a, b) for(int i = a; i < b; i++)
#define fi first
#define se second
#define pll pair<ll, ll>
#define mpp make_pair
#define ALL(v) v.begin(), v.end()
#define ALLR(v) v.rbegin(), v.rend()
#define ld long double

const ll mxn = 4e5 + 5, inf = 1e9, mod = 998244353, sq = 800, mxv = 1e6 + 5, pr = 37, mod2 = 1e9 + 9, mod3 = 998244353;
//const int x[4] = {0, -1, 0, 1};
 
const int x[4] = {1, -1, 0, 0};
const int y[4] = {0, 0, 1, -1};
//const char dir[4] = {'D', 'U', 'R', 'L'};
//mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
int n;
bool same[4005][4005]; //can be on the same path
bool has[2005][4005];
bool vis[4005];
vt<int>g[mxn + 1];
vt<pii>adj[mxn + 1];
int low[mxn + 1], num[mxn + 1], id[mxn + 1], idscc = 0, tt = 0, in[mxn + 1];
bool lol[mxn + 1];
bool used[mxn + 1];
vt<int>st;

int rev(int x, int mid){
    return(x + 2 * mid);
}
void dfs2(int s){
   
    num[s] = low[s] = ++tt;
    st.pb(s);
    for(auto i: g[s]){
        if(used[i])continue;
        if(!num[i]){
            dfs2(i); low[s] = min(low[s], low[i]);
        }else{
            low[s] = min(low[s], num[i]);
        }
    }
    if(low[s] == num[s]){
       
        int v;
        idscc++;
        do{
            v = st.back(); st.pop_back(); id[v] = idscc; used[v] = 1;
        }while(v != s);
    }
}

bool ck(int mid){
    for(int i = 0; i < 4 * mid; i++){
        g[i].clear(); num[i] = low[i] = id[i] = used[i] = 0;
    }
    st.clear(); 
    idscc = tt = 0;
    for(int i = 0; i < 2 * mid; i += 2){
        g[rev(i, mid)].pb(i + 1); g[rev(i + 1, mid)].pb(i);
    }
    for(int i = 0; i < 2 * mid; i++){
        for(int j = i + 1; j < 2 * mid; j++){
            if(!same[i][j]){
               // cout << i << " " << j << "\n";
                g[i].pb(rev(j, mid)); g[j].pb(rev(i, mid));
                
            }
        }
    }
    for(int i = 0; i < 4 * mid; i++){
        if(!num[i]){
            dfs2(i);
        }
    }
    for(int i = 0; i < 2 * mid; i++){
        if(id[i] == id[i + 2 * mid])return(0);
    }
    return(1);
}
void solve(){       
    cin >> n;
    for(int i = 0; i < 2 * n; i++){
        int u, v; cin >> u >> v;
        adj[u].pb(mpp(v, i)); in[v]++;
    }
    for(int i = 1; i <= n; i++){
        sort(ALL(adj[i]));
    }
    queue<int>q;
    for(int i = 1; i <= n; i++){
        if(in[i] == 0)q.push(i);
    }
    while(!q.empty()){
        int nw = q.front(); q.pop();
        for(auto [v, eid]: adj[nw]){
            has[v][eid] = 1;
            for(int j = 0; j < 2 * n; j++){
                same[eid][j] = same[j][eid] = has[nw][j];
                has[v][j] |= has[nw][j];
            }
            in[v]--;
            if(in[v] == 0)q.push(v);
        }
    }
    int l = 1, r = n, ans = 0;
    
    
    while(l <= r){
        int mid = (l + r) >> 1;
        if(ck(mid)){
            ans = mid; l = mid + 1;
        }else{
            r = mid - 1;
        }
    }
    
    cout << ans;
}
 
signed main(){
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    //freopen("ARRAY.inp", "r", stdin);
    //freopen("ARRAY.out", "w", stdout);
    
    int tt; tt = 1;
    while(tt--){
        solve();
    }
    return(0);
}