#include <bits/stdc++.h>
#define fi first
#define se second
#define pb push_back
#define sz(a) (ll)a.size()
#define rep(i,n) for(ll i=0;i<n;i++)
#define drep(i,n) for(ll i=n-1;i>=0;i--)
#define vec(...) vector<__VA_ARGS__>
#define _3aILRlt ios_base::sync_with_stdio(false),cin.tie(0),cout.tie(0)
using namespace std;
typedef long long ll;
using pii=pair<ll,ll>;
using tpii=pair<pii,ll>;
using vi=vector<ll>;

int main(){
_3aILRlt;
	ll n,d;
	cin>>n>>d;
	vec(pii) a(n);
	rep(i,n){
		cin>>a[i].fi>>a[i].se;
	}
	auto f=[&](ll x)->bool{
		ll now=0;
		vi usd(n,0);
		rep(_d,d){
			ll maxima=-1e9,id=-1;
			rep(i,n){
				if(usd[i]) continue;
				if(now-a[i].fi>=x){
					if(-a[i].fi+a[i].se>maxima){
						maxima=-a[i].fi+a[i].se;
						id=i;
					}
				}
			}
			if(id==-1){
				return false;
			}
			usd[id]=1;
			now+=maxima;
		}
		return true;
	};
	ll l=-1e9-1,r=1e9+1,c=-1;
	while(l<=r){
		ll m=(l+r)/2;
		if(f(m)){
			c=m;
			l=m+1;
		}else{
			r=m-1;
		}
	}	
	cout<<c<<"\n";
//	
	return 0;
}