#include<iostream>
#include<set>
#include<vector>
#include<algorithm>
using namespace std;
int N,D;
main()
{
	cin>>N>>D;
	vector<pair<int,pair<int,int> > >PQ(N);
	for(int i=0;i<N;i++)
	{
		int p,q;cin>>p>>q;
		PQ[i]=make_pair(p,make_pair(p-q,i));
	}
	sort(PQ.begin(),PQ.end());
	int L=-1e9,R=1;
	while(R-L>1)
	{
		int mid=(L+R)/2;
		int now=0;
		int id=0;
		multiset<pair<int,int> >S;
		int prv=-1;
		bool ok=true;
		for(int d=0;d<D;d++)
		{
			while(id<N&&PQ[id].first<=now-mid)
			{
				S.insert(PQ[id].second);
				id++;
			}
			while(id>0&&PQ[id-1].first>now-mid)
			{
				id--;
				S.erase(S.find(PQ[id].second));
			}
			if(S.empty())
			{
				ok=false;
				break;
			}
			auto it=S.begin();
			if(it->second!=prv)
			{
				now-=it->first;
				prv=it->second;
			}
			else
			{
				it++;
				if(it==S.end())
				{
					ok=false;
					break;
				}
				now-=it->first;
				prv=it->second;
			}
		}
		if(ok)L=mid;
		else R=mid;
	}
	cout<<L<<endl;
}