#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const ll MOD = 1000000007;
using P = pair<int,int>;
#define rep(i, n) for(int i = 0; i < n; i++)
#define all(x) (x).begin(),(x).end()
template<class T>bool chmin(T&a, const T&b){if(a>b){a=b;return 1;}return 0;}
template<class T>bool chmax(T&a, const T&b){if(a<b){a=b;return 1;}return 0;}
int main(){
    int n,d;
    cin >> n >> d;
    vector<P> pq(n);
    rep(i,n){
        cin >> pq[i].first >> pq[i].second;
        pq[i].second -= pq[i].first;
    }
    sort(all(pq));
    int be = -100000001, en = 0;
    while(en-be>1){
        int md = (be+en)/2, now = md + pq[0].first;
        int pre = 0;
        rep(i,d-1){
            int flag = -1, mi = MOD;
            rep(j,n){
                if(j==pre) continue;
                if(chmin(mi,pq[j].second)) flag = j;
            }
            //cout << mi << " ";
            pre = flag;
            now -= mi;
        }
        //cout << endl;
        if(now <= 0) be = md;
        else en = md;
    }
    cout << be << endl;
    return 0;
}