区间覆盖

#include <iostream>
#include <algorithm>
using namespace std;
const int N=10010;
int n;
struct Range
{
int l,r;
bool operator<(const Range&W) const
{
return l<W.l;
}
}range[N];
int main()
{
int start,end;
cin>>start>>end;
cin>>n;
for(int i=0;i<n;i++)
{
int l,r;
cin>>l>>r;
range[i]={l,r};
}
sort(range,range+n);
int res=0;
bool success=false;
for(int i=0;i<n;i++)
{
int j=i,r=-2e9;
while(j<n&&range[j].l<=start)
{
r=max(r,range[j].r);
j++;
}
if(r<start)
{
res=-1;
break;
}
res++;
if(r>=end)
{
success=true;
break;
}
start=r;
i=j-1;
}
if(!success) res=-1;
cout<<res<<endl;
}