#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
#define st first
#define nd second
#define pb push_back
#define all(a) a.begin(),a.end()
#define rep(a,b) for(int a = 0;a<b;a++)
const int inf = 1e9;
const ll infl = 1e18;
int rang[200000];
int main()
{
int n;
cin>>n;
rep(i,n)
{
int a,b;
cin>>a>>b;
rang[i] = b-1;
}
int uss;
cin>>uss>>uss;
int r = 0;
int mx = rang[0];
int ans = 0;
while(r < n-1)
{
int prev = r;
r = mx;
for(int i = prev + 1;i<=r;i++)
{
mx = max(mx,rang[i]);
}
ans++;
if(prev == r)
{
cout<<-1<<"\n";
return 0;
}
}
cout<<ans<<"\n";
}
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