using System;
using System.Collections.Generic;
using System.Collections;
using System.Collections.Specialized;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.IO;
using System.Reflection;
using static System.Math;
using System.Numerics;
using System.Threading;
using System.Runtime.CompilerServices;
using System.Diagnostics;
using nint=System.Int64;
static class Program{
	const int mod=(int)1e9+7;
	static void Main(){
		Sc sc=new Sc();
		var (n,d)=sc.Tp2<int>();
		var avl=new Avl(true,true);
		var inf=long.MaxValue/3000;
		for(int i = 0;i<n;i++) {
			var e=sc.La;
			avl.Ud(-e[0]*2000-i,e[1]-e[0]);
		}
		long lb = -inf, ub = 2, mid = 0;
		while(lb+1!=ub) {
			mid=(ub+lb)/2;
			if(!Fu(mid)) { ub=mid; }
			else { lb=mid; }
		}
		bool Fu(long m) {
			var p=0L;
			var z=0L;
			var c=-inf;
			for(int i = 0;i<d;i++) {
				var e=avl.Rd((m-p)*2000-1999,c-1);
				var f=avl.Rd(Max((m-p)*2000-1999,c+1),inf);
				if(e==avl.du&&f==avl.du){return false;}
				if(e.d<f.d){e=f;}
				c=e.n;
				z=Min(z,p+c/2000);
				p+=e.d;
			}
			return z>=m;
		}
		Console.WriteLine("{0}",lb);
		//Console.WriteLine("{0}",string.Join(" ",a));
	}
}
public class Avl{
	public class Nd{
		public int h=1,c=1;
		public readonly nint n;
		public long d;
		public Nd l,r,nx;
		public Nd(nint n,long d,Nd du){this.n=n;this.d=d;nx=this;l=du;r=du;}
		public override string ToString()=>"n:"+n.ToString()+" d:"+d.ToString();
	}
	private Func<long,long,bool> compare;
	private Action<Nd> compare2;
	public Nd root,du;
	public int cnt=0;
	public bool bb;
	public Avl(bool bo,bool bb){
		if(bo){
			du=new Nd(nint.MinValue,long.MinValue,du);
			compare=CoMax;
			compare2=CoMax;
		}
		else{
			du=new Nd(nint.MinValue,long.MaxValue,du);
			compare=CoMin;
			compare2=CoMin;
		}
		this.bb=bb;
		du.h=0;du.c=0;
		root=du;
	}
	public Avl(Avl z){
		if(z.du.d==long.MinValue){
			du=new Nd(nint.MinValue,long.MinValue,du);
			compare=CoMax;
			compare2=CoMax;
		}
		else{
			du=new Nd(nint.MinValue,long.MaxValue,du);
			compare=CoMin;
			compare2=CoMin;
		}
		bb=z.bb;
		du.h=0;du.c=0;
		root=Fu(z.root);
		cnt=z.cnt;
		Nd Fu(Nd nd){
			if(nd.h==0){return du;}
			Nd mn=new Nd(nd.n,nd.d,du);
			mn.h=nd.h;
			mn.c=nd.c;
			mn.l=Fu(nd.l);
			mn.r=Fu(nd.r);
			compare2(mn);
			return mn;
		}
	}
	private bool CoMax(long a,long b){return a>=b;}
	private bool CoMin(long a,long b){return a<=b;}
	private void CoMax(Nd t){
		t.nx=t;
		if(t.nx.d<t.l.nx.d){t.nx=t.l.nx;}
		if(t.nx.d<t.r.nx.d){t.nx=t.r.nx;}
	}
	private void CoMin(Nd t){
		t.nx=t;
		if(t.nx.d>t.l.nx.d){t.nx=t.l.nx;}
		if(t.nx.d>t.r.nx.d){t.nx=t.r.nx;}
	}
	public void Ud(nint n,long d){
		if(root==du){root=new Nd(n,d,du);cnt++;}
		else if(root.n==n){
			if(bb){root.d+=d;}
			else{root.d=d;}
			compare2(root);
		}
		else if(Fu(root,n,d)){
			if(Abs(root.l.h-root.r.h)>1){root=Rotate(root);}
			cnt++;
		}
	}
	private bool Fu(Nd t,nint n,long d){
		bool bo=false;
		if(t.n>n){
			if(t.l!=du){
				if(t.l.n==n){
					if(bb){t.l.d+=d;}
					else{t.l.d=d;}
					compare2(t.l);
				}
				else if(bo=Fu(t.l,n,d)){
					t.c++;
					if(Abs(t.l.l.h-t.l.r.h)>1){t.l=Rotate(t.l);}
					t.h=Max(t.l.h,t.r.h)+1;
				}
				compare2(t);
			}
			else{
				t.l=new Nd(n,d,du);
				bo=true;
				t.c++;
				t.h=Max(t.l.h,t.r.h)+1;
				t.nx=compare(t.nx.d,t.l.d)?t.nx:t.l;
			}
		}
		else{
			if(t.r!=du){
				if(t.r.n==n){
					if(bb){t.r.d+=d;}
					else{t.r.d=d;}
					compare2(t.r);
				}
				else if(bo=Fu(t.r,n,d)){
					t.c++;
					if(Abs(t.r.l.h-t.r.r.h)>1){t.r=Rotate(t.r);}
					t.h=Max(t.l.h,t.r.h)+1;
				}
				compare2(t);
			}
			else{
				t.r=new Nd(n,d,du);
				bo=true;
				t.c++;
				t.h=Max(t.l.h,t.r.h)+1;
				t.nx=compare(t.nx.d,t.r.d)?t.nx:t.r;
			}
		}
		return bo;
	}
	private Nd Rotate(Nd t){
		Nd nd=du;
		if(t.l.h>t.r.h){
			if(t.l.l.h>t.l.r.h){
				nd=t.l;t.l=t.l.r;nd.r=t;
				Ft(t);
			}
			else{
				nd=t.l.r;
				t.l.r=nd.l;nd.l=t.l;
				Ft(t.l);
				t.l=nd.r;nd.r=t;
				Ft(t);
			}
		}
		else{
			if(t.r.l.h>t.r.r.h){
				nd=t.r.l;
				t.r.l=nd.r;nd.r=t.r;
				Ft(t.r);
				t.r=nd.l;nd.l=t;
				Ft(t);
			}
			else{
				nd=t.r;t.r=t.r.l;nd.l=t;
				Ft(t);
			}
		}
		Ft(nd);
		return nd;
	}
	private void Ft(Nd t){
		t.h=Max(t.l.h,t.r.h)+1;
		t.c=t.l.c+t.r.c+1;
		compare2(t);
	}
	public bool Dl(nint n){
		if(cnt==0){return false;}
		Nd t=root;
		if(t.n==n){
			root=Fd2(root);
			if(cnt==0){return true;}
			else if(Abs(root.l.h-root.r.h)>1){root=Rotate(root);}
			else{Ft(root);}
			return true;
		}
		bool bo=Fd1(root,n);
		if(Abs(root.l.h-root.r.h)>1){root=Rotate(root);}
		else{Ft(root);}
		return bo;
	}
	private bool Fd1(Nd t,nint n){
		if(t.n>n){
			if(t.l==du){return false;}
			else if(t.l.n!=n){
				bool bo=Fd1(t.l,n);
				if(Abs(t.l.l.h-t.l.r.h)>1){t.l=Rotate(t.l);}
				else{Ft(t.l);}
				return bo;
			}
			else{
				t.l=Fd2(t.l);
				Ft(t);
				return true;
			}
		}
		else{
			if(t.r==du){return false;}
			else if(t.r.n!=n){
				bool bo=Fd1(t.r,n);
				if(Abs(t.r.l.h-t.r.r.h)>1){t.r=Rotate(t.r);}
				else{Ft(t.r);}
				return bo;
			}
			else{
				t.r=Fd2(t.r);
				Ft(t);
				return true;
			}
		}
	}
	private Nd Fd2(Nd dn){
		cnt--;
		if(dn.l==du){return dn.r;}
		if(dn.r==du){return dn.l;}
		Nd u=dn.l;
		if(u.r==du){
			u.r=dn.r;
			Ft(u);
			return u;
		}
		u=Fd3(u,dn);
		if(Abs(dn.l.l.h-dn.l.r.h)>1){u.l=Rotate(dn.l);}
		else{Ft(dn.l);}
		Ft(u);
		return u;
	}
	private Nd Fd3(Nd u,Nd dn){
		if(u.r.r!=du){
			Nd v=Fd3(u.r,dn);
			if(Abs(u.r.l.h-u.r.r.h)>1){u.r=Rotate(u.r);}
			else{Ft(u.r);}
			return v;
		}
		else{
			Nd v=u.r;u.r=u.r.l;v.l=dn.l;v.r=dn.r;
			return v;
		}
	}
	public Nd Hs(nint n){
		Nd t=root;
		if(cnt==0){return du;}
		if(t.n==n){return t;}
		Nd a=du;
		while(true){
			if(t.n>n){
				if(t.l==du){break;}
				else if(t.l.n!=n){t=t.l;}
				else{a=t.l;break;}
			}
			else{
				if(t.r==du){break;}
				else if(t.r.n!=n){t=t.r;}
				else{a=t.r;break;}
			}
		}
		return a;
	}
	public Nd Ra(int n){return cnt>=n&&cnt!=0?Fr(root,n):du;}
	private Nd Fr(Nd t,int n){
		if(t.l.c>n-1){return Fr(t.l,n);}
		if(t.l.c<n-1){return Fr(t.r,n-t.l.c-1);}
		return t;
	}
	public Nd Lb(nint n){return cnt!=0?Flb(root,n):du;}
	private Nd Flb(Nd t,nint n){
		if(t.n<n){
			if(t.r==du){return du;}
			return Flb(t.r,n);
		}
		if(t.n>n){
			if(t.l==du){return t;}
			Nd u=Flb(t.l,n);
			return u==du?t:u;
		}
		return t;
	}
	public Nd Ub(nint n){return cnt!=0?Fub(root,n):du;}
	private Nd Fub(Nd t,nint n){
		if(t.n>n){
			if(t.l==du){return du;}
			return Fub(t.l,n);
		}
		if(t.n<n){
			if(t.r==du){return t;}
			Nd u=Fub(t.r,n);
			return u==du?t:u;
		}
		return t;
	}
	public int Rg(nint a,nint b){return cnt!=0?Frg(root,a,b,false,false):0;}
	private int Frg(Nd t,nint a,nint b,bool l,bool r){
		if(t.h==0){return 0;}
		if(l&&r){return t.c;}
		if(t.n<a){return Frg(t.r,a,b,t.n>=a,r);}
		if(t.n>b){return Frg(t.l,a,b,l,t.n<=b);}
		return Frg(t.l,a,b,l,t.n<=b)+Frg(t.r,a,b,t.n>=a,r)+1;
	}
	public Nd Rd(nint a,nint b){return cnt!=0?Frd(root,a,b,false,false):du;}
	private Nd Frd(Nd t,nint a,nint b,bool l,bool r){
		if(t.h==0){return du;}
		if(l&&r){return t.nx;}
		if(t.n<a){return Frd(t.r,a,b,t.n>=a,r);}
		if(t.n>b){return Frd(t.l,a,b,l,t.n<=b);}
		var e1=Frd(t.l,a,b,l,t.n<=b);
		var e2=Frd(t.r,a,b,t.n>=a,r);
		e1=compare(e1.d,e2.d)?e1:e2;
		return compare(t.d,e1.d)?t:e1;
	}
	public Nd Gl(nint a,nint b,long d){return cnt==0?du:Fgl(root,a,b,d);}
	private Nd Fgl(Nd t,nint a,nint b,long d){
		Nd p=du;
		if(a<=t.n&&t.n<=b&&compare(t.d,d)){p=t;}
		if(a<=t.n&&compare(t.l.nx.d,d)){
			var z=Fgl(t.l,a,b,d);
			if(z!=du){p=z;}
		}
		if(p==du&&t.n<=b&&compare(t.r.nx.d,d)){p=Fgl(t.r,a,b,d);}
		return p;
	}
	public Nd Gr(nint a,nint b,long d){return cnt==0?du:Fgr(root,a,b,d);}
	private Nd Fgr(Nd t,nint a,nint b,long d){
		Nd p=du;
		if(a<=t.n&&t.n<=b&&compare(t.d,d)){p=t;}
		if(t.n<=b&&compare(t.r.nx.d,d)){
			var z=Fgr(t.r,a,b,d);
			if(z!=du){p=z;}
		}
		if(p==du&&a<=t.n&&compare(t.l.nx.d,d)){p=Fgr(t.l,a,b,d);}
		return p;
	}
	public void En(Action<Nd> f){if(cnt>0){Fen(root,f);}}
	private void Fen(Nd t,Action<Nd> f){
		f(t);
		if(t.l!=du){Fen(t.l,f);}
		if(t.r!=du){Fen(t.r,f);}
	}
}
public class Sc{
	public int I{get{return int.Parse(Console.ReadLine());}}
	public long L{get{return long.Parse(Console.ReadLine());}}
	public double D{get{return double.Parse(Console.ReadLine());}}
	public string S{get{return Console.ReadLine();}}
	public int[] Ia{get{return Array.ConvertAll(Console.ReadLine().Split(),int.Parse);}}
	public long[] La{get{return Array.ConvertAll(Console.ReadLine().Split(),long.Parse);}}
	public double[] Da{get{return Array.ConvertAll(Console.ReadLine().Split(),double.Parse);}}
	public string[] Sa{get{return Console.ReadLine().Split();}}
	public object[] Oa{get{return Console.ReadLine().Split();}}
	public int[] Ia2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),int.Parse);}}
	public int[] Ia3(string a,string b){return Array.ConvertAll((a+Console.ReadLine()+b).Split(),int.Parse);}
	public int[] Ia3(int a){return Array.ConvertAll((Console.ReadLine()+" "+a.ToString()).Split(),int.Parse);}
	public long[] La2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),long.Parse);}}
	public long[] La3(string a,string b){return Array.ConvertAll((a+Console.ReadLine()+b).Split(),long.Parse);}
	public long[] La3(int a){return Array.ConvertAll((Console.ReadLine()+" "+a.ToString()).Split(),long.Parse);}
	public double[] Da2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),double.Parse);}}
	public double[] Da3(string a,string b){return Array.ConvertAll((a+Console.ReadLine()+b).Split(),double.Parse);}
	public T[] Arr<T>(int n,Func<T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f();}return a;}
	public T[] Arr<T>(int n,Func<int,T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(i);}return a;}
	public T[] Arr<T>(int n,Func<string[],T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(Console.ReadLine().Split());}return a;}
	public T[] Arr<T>(int n,Func<int,string[],T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(i,Console.ReadLine().Split());}return a;}
	public (T,T) Tp2<T>(){var s=Console.ReadLine().Split();return (Ct<T>(s[0]),Ct<T>(s[1]));}
	public (T,T,T) Tp3<T>(){var s=Console.ReadLine().Split();return (Ct<T>(s[0]),Ct<T>(s[1]),Ct<T>(s[2]));}
	public (T,T,T,T) Tp4<T>(){var s=Console.ReadLine().Split();return (Ct<T>(s[0]),Ct<T>(s[1]),Ct<T>(s[2]),Ct<T>(s[3]));}
	public (T1,T2) Tp2<T1,T2>(){var s=Console.ReadLine().Split();return (Ct<T1>(s[0]),Ct<T2>(s[1]));}
	public (T1,T1,T2) Tp3<T1,T2>(){var s=Console.ReadLine().Split();return (Ct<T1>(s[0]),Ct<T1>(s[1]),Ct<T2>(s[2]));}
	private T Ct<T>(string s){return (T)Convert.ChangeType(s,typeof(T));}
}