#include<bits/stdc++.h>
#define LL long long
#define LLL __int128
#define ldb long double
using namespace std;
typedef pair<int,int> PII;
#define gc() getchar()
#define pc(x) putchar(x)
template<class T>inline T read(){
    T x=0;char ch;bool f=1;
    while(!isdigit(ch=gc()))if(ch=='-')f^=1;
    do x=(x<<1)+(x<<3)+(ch^48);while(isdigit(ch=gc()));
    return f?x:-x;
}
#define read() read<int>()
const int N=1e5+5,Mod=998244353;
int n,m,flg;
PII A[N];
int f[4][N];
int pre[N],nxt[N],B[N<<1],F[N<<3],G[N<<3];
void build(int p,int l,int r){
	F[p]=n+1,G[p]=0;
	if(l==r)return;
	int mid=(l+r)>>1;
	build(p<<1,l,mid),build(p<<1|1,mid+1,r);
}
void updmin(int p,int l,int r,int x,int y){
	F[p]=min(F[p],y);
	if(l==r)return;
	int mid=(l+r)>>1;
	if(x<=mid)updmin(p<<1,l,mid,x,y);
	else updmin(p<<1|1,mid+1,r,x,y);
}
void updmax(int p,int l,int r,int x,int y){
	G[p]=max(G[p],y);
	if(l==r)return;
	int mid=(l+r)>>1;
	if(x<=mid)updmax(p<<1,l,mid,x,y);
	else updmax(p<<1|1,mid+1,r,x,y);
}
int qmin(int p,int l,int r,int x){
	if(x<=l)return F[p];
	int mid=(l+r)>>1;
	if(x>mid)return qmin(p<<1|1,mid+1,r,x);
	return min(qmin(p<<1,l,mid,x),F[p<<1|1]);
}
int qmax(int p,int l,int r,int x){
	if(r<=x)return G[p];
	int mid=(l+r)>>1;
	if(x<=mid)return qmax(p<<1,l,mid,x);
	return max(G[p<<1],qmax(p<<1|1,mid+1,r,x));
}
signed main(){
	n=read();
	for(int i=1;i<=n;++i){
		int l=B[++m]=read(),r=B[++m]=read();
		A[i]=PII(l,r);
	}
	sort(A+1,A+n+1),sort(B+1,B+m+1),m=unique(B+1,B+m+1)-B-1;
	for(int i=2;i<=n;++i)if(A[i-1].second>=A[i].first)flg=1;
	if(!flg)return puts("impossible"),0;
	build(1,1,m);
	for(int i=1;i<=n;++i){
		A[i].first=lower_bound(B+1,B+m+1,A[i].first)-B;
		A[i].second=lower_bound(B+1,B+m+1,A[i].second)-B;
		if(A[i].first!=1)pre[i]=qmax(1,1,m,A[i].first-1);
		updmax(1,1,m,A[i].second,i);
	}
	for(int i=1;i<=n;++i){
		nxt[i]=qmin(1,1,m,A[i].first);
		updmin(1,1,m,A[i].second,i);
		if(nxt[i]==n+1)nxt[i]=i;
	}
	memset(f,0x3f,sizeof(f));
	for(int j=0;j<=3;++j)for(int i=1;i<=n;++i)
		if(!pre[i])f[j][i]=1;
		else for(int k=0,p=i;k<=j;++k)
			if(!pre[p=nxt[p]]){
				f[j][i]=1;
				break;
			}
			else f[j][i]=min(f[j][i],f[j-k][pre[p]]+1);
	puts(f[1][n]<f[0][n]?"1":(f[2][n]<f[0][n]?"2":(f[3][n]<f[0][n]?"3":"impossible")));
    return 0;
}
/*
 
*/