//GIVE ME AC!!!!!!!!!!!!!!!!!
#pragma GCC target("avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include<bits/stdc++.h>
using namespace std;
using ll=long long;
using ld=long double;
using vl=vector<ll>;
using vi=vector<int>;
using vs=vector<string>;
using vc=vector<char>;
using vvl=vector<vl>;
using P=pair<ll,ll>;
using vvc=vector<vc>;
using vd=vector<double>;
using vp=vector<P>;
using vb=vector<bool>;
#define overload4(_1,_2,_3,_4,name,...) name
#define overload3(_1,_2,_3,name,...) name
#define rep1(a) for(decltype(a) i=0;i<a;i++)
#define rep2(i,a) for(decltype(a) i=0;i<a;i++)
#define rep3(i,a,b) for(decltype(a) i=a;i<b;i++)
#define rep4(i,a,b,c) for(decltype(a) i=a;i<b;i+=c)
#define rep(...) overload4(__VA_ARGS__, rep4, rep3, rep2, rep1)(__VA_ARGS__)
#define rrep1(a) for(decltype(a) i=(a)-1;i>=0;i--)
#define rrep2(i,a) for(decltype(a) i=(a)-1;i>=0;i--)
#define rrep3(i,a,b) for(decltype(a) i=(b)-1;i>=(a);i--)
#define rrep(...) overload3(__VA_ARGS__,rrep3,rrep2,rrep1)(__VA_ARGS__)
#define all1(i) begin(i),end(i)
#define all2(i,a) begin(i),begin(i)+a
#define all3(i,a,b) begin(i)+a,begin(i)+b
#define all(...) overload3(__VA_ARGS__,all3,all2,all1)(__VA_ARGS__)
#define rall(n) (n).rbegin(),(n).rend()
#define pb push_back
#define eb emplace_back
#define MtSaka ios::sync_with_stdio(0);cin.tie(0);cout<<fixed<<setprecision(12)
#define max_(a) *max_element(all(a))
#define min_(a) *min_element(all(a))
#define INT(...) int __VA_ARGS__;scan(__VA_ARGS__)
#define LL(...) ll __VA_ARGS__;scan(__VA_ARGS__)
#define STR(...) string __VA_ARGS__;scan(__VA_ARGS__)
#define CHR(...) char __VA_ARGS__;scan(__VA_ARGS__)
#define DBL(...) double __VA_ARGS__;scan(__VA_ARGS__)
#define LD(...) ld __VA_ARGS__;scan(__VA_ARGS__)
const int dx[8]={1,0,-1,0,1,-1,-1,1};
const int dy[8]={0,1,0,-1,1,1,-1,-1};
const ll inf=2e18;
const ll MOD=1000000007;
const ll mod=998244353;
const double pi=acos(-1);
template<typename T1,typename T2 >
ostream &operator<<(ostream&os,const pair<T1,T2>&p){os<<p.first<<" "<<p.second;return os;}
template<typename T1,typename T2>
istream &operator>>(istream&is,pair<T1,T2>&p){is>>p.first>>p.second;return is;}
template<typename T>
ostream &operator<<(ostream&os,const vector<T>&v){for(int i=0;i<(int)v.size();i++){os<<v[i]<<(i+1!=v.size()?" ":"");}return os;}
template<typename T>
istream &operator>>(istream&is,vector<T>&v){for(T &in:v){is>>in;}return is;}
void scan(){}
template<class Head,class... Tail>
void scan(Head&head,Tail&... tail){cin>>head;scan(tail...);}
template<class T>
void print(const T &t){cout<<t<<'\n';}
template<class Head, class... Tail>
void print(const Head &head, const Tail &... tail){cout<<head<<' ';print(tail...);}
template<class... T>
void fin(const T &... a){print(a...);exit(0);}
template<typename T>
T sum_(vector<T>a){return accumulate(all(a),T(0));}
template<typename T1,typename T2>
inline bool chmax(T1&a,T2 b){return a<b&&(a=b,true);}
template<typename T1,typename T2>
inline bool chmin(T1&a,T2 b){return a>b&&(a=b,true);}
int main(){
  MtSaka;
  LL(n,d);
  vp p(n);
  scan(p);
  ll ng=0,ok=1e9;
  while(ok-ng>1){
    ll mid=(ok+ng)/2;
    //print(mid);
    ll s=0;
    ll before=-1;
    bool f=true;
    rep(i,d){
      ll a=inf;
      ll now=-1;
      rep(j,n){
        if(mid-s<p[j].first||j==before)continue;
        if(chmin(a,s+p[j].first-p[j].second))now=j;
      }
      //print(s,now);
      if(now==-1){
        f=false;
        break;
      }
      s=a;
      before=now;
    }
    if(f)ok=mid;
    else ng=mid;
  }
  print(-ok);
}