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

struct interval
{
	int in;
	int sf;
};

bool cmp(interval a, interval b)
{
	return a.in<b.in;
}

interval v[1000];

int max(int a, int b)
{
	if(a>b)return a;
	if(b>a)return b;
	return a;
}

int main()
{
	freopen("acoperire.in","r",stdin);
	freopen("acoperire.out","w",stdout);
	int st,dr,n;
	scanf("%d%d",&st,&dr); // capetele intervalului
	scanf("%d",&n); // nr intervale mici
	for(int i=1;i<=n;i++)
		scanf("%d%d",&v[i].in,&v[i].sf); // intervale mici
	int reper=st, departe=st-1,nr=1;
	sort(v+1,v+n+1,cmp);
	bool ok=0;
	for(int i=1;i<=n;i++)
	{
		if(v[i].in<=reper)
			departe=max(departe,v[i].sf);
		else
			if(v[i].in>departe)
			{
				printf("-1");
				ok=1;
				break;
			}
			else
			{
				nr++;
				reper=departe;
				departe=v[i].sf;
			}
		if(departe>=dr)
			break;
	}
	if(ok==0)
		if(v[n].sf<st)
		{
			printf("-1");
			ok=1;
		}
	if(ok==0)
		printf("%d",nr);
	return 0;
}