#include<bits/stdc++.h>
using namespace std;
typedef long long ll;

typedef struct{
    ll p;
    ll q;
    ll num;
} CELL;

int cmp(CELL a, CELL b){
    if(a.q - a.p > b.q - b.p){
        return 1;
    }else{
        return 0;
    }
}

int main(){
    ll n,d,t,l = -100000000,r = 0,m,tm;
    CELL c[1000] = {};
    bool f;
    cin >> n >> d;
    for(int i = 0; i < n; i++){
        cin >> c[i].p >> c[i].q;
        c[i].num = i;
    }
    sort(c,c+n,cmp);
    while(l < r){
        m = (l+r)/2;
        tm = 0;
        t = -1;
        f = true;
        for(int i = 0; i < d; i++){
            for(int j = 0; j < n; j++){
                if(tm >= m + c[j].p && t != j){
                    t = j;
                    tm += c[j].q - c[j].p;
                    f = false;
                    break;
                }
            }
            if(f){
                if(r == m){
                    r = l;
                }else{
                    r = m;
                }
                break;
            }
        }
        if(!f){
            if(r == m){
                l = r;
            }else{
                l = m;
            }
        }
    }
    cout << l - 1;
}