#include <cstdio>
#include <vector>
#include <map>
#include <algorithm>
using namespace std;

#define N 10
int n;

struct S {
  int vs[N][2];
};

int operator<(const S& a, const S& b) {
  for (int i=0;i<n;++i) {
    int av_min=min(a.vs[i][0],a.vs[i][1]);
    int av_max=max(a.vs[i][0],a.vs[i][1]);
    int bv_min=min(b.vs[i][0],b.vs[i][1]);
    int bv_max=max(b.vs[i][0],b.vs[i][1]);
    if (av_min<bv_min) return 1;
    if (av_min>bv_min) return 0;
    if (av_max<bv_max) return 1;
    if (av_max>bv_max) return 0;
  }
  return 0;
}
map<S,int> memo;

void dump(S& cur) {
  puts("");
  for (int i=0;i<n;++i) printf("%2d ",cur.vs[i][0]+1);
  puts("");
  for (int i=0;i<n;++i) printf("%2d ",cur.vs[i][1]+1);
  puts("");
}

int get_lb1(S& cur, int st, int en) {
  int res=0;
  for (int i1=st;i1<en;++i1)
    for (int j1=0;j1<2;++j1)
      for (int i2=i1+1;i2<en;++i2)
				for (int j2=0;j2<2;++j2) {
					if (cur.vs[i1][j1]==cur.vs[i2][j2]) {
						++res;
					} else if (cur.vs[i1][j1]>cur.vs[i2][j2]) {
						res+=2;
					}
				}
	for (int i=st;i<en;++i) {
		if (cur.vs[i][0]!=cur.vs[i][1]) ++res;
		else {
			int ok=1;
			for (int j=st;j<i&&ok;++j) 
				if (cur.vs[j][0]>i||cur.vs[j][1]>i)
					ok=0;
			if (!ok) res+=2;
		}
	}
  return (res+5)/6;
}

int get_lb2(S& cur, int st, int en) {
  int res=0;
  for (int i=st;i<en;++i)
    for (int j=0;j<2;++j)
      res+=abs(cur.vs[i][j]-i);
  return (res+1)/2;
}

int solve(S& cur, int cnt, int ub, vector<S>& log) {
	int st=0,en=n;
	while(st<n&&cur.vs[st][0]==st&&cur.vs[st][1]==st) ++st;
	while(en-1>=0&&cur.vs[en-1][0]==en-1&&cur.vs[en-1][1]==en-1) --en;
	if (st>=en) return 1;

  int lb=cnt+max(get_lb1(cur,st,en),get_lb2(cur,st,en));
  if (lb>ub) return 0;

  map<S,int>::iterator it=memo.find(cur);
  if (it!=memo.end()&&it->second<=cnt) return 0;
  memo[cur]=cnt;

  for (int i=st;i<en-1;++i) {
    for (int j1=0;j1<(cur.vs[i][0]!=cur.vs[i][1]?2:1);++j1) {
			if (cur.vs[i][j1]<cur.vs[i][1-j1]) continue;
      for (int j2=0;j2<(cur.vs[i+1][0]!=cur.vs[i+1][1]?2:1);++j2) {
				if (cur.vs[i+1][j2]>cur.vs[i+1][1-j2]) continue;
				if (cur.vs[i][j1]<=cur.vs[i+1][j2]) continue;
				swap(cur.vs[i][j1],cur.vs[i+1][j2]);
				log.push_back(cur);
				if (solve(cur,cnt+1,ub,log)) return 1;
				log.pop_back();
				swap(cur.vs[i][j1],cur.vs[i+1][j2]);
      }
		}
  }

  return 0;
}

int main() {
  scanf("%d",&n);
	S start;
	for (int i=0;i<n;++i) {
		scanf("%d%d",&start.vs[i][0],&start.vs[i][1]);
		--start.vs[i][0];
		--start.vs[i][1];
	}

	int ans=0;
	vector<S> log;
	log.push_back(start);
	while(1) {
		memo.clear();
		if (solve(start,0,ans,log)) break;
		++ans;
	}
	printf("%d\n",ans);
}