#include <bits/stdc++.h>
using namespace std;
//#include<atcoder/all>
//using namespace atcoder;
using ll = long long int;
using ull = unsigned long long int;
using ld = long double;
constexpr ll MAX = 2000000000000000000;
constexpr ld PI = 3.14159265358979;
constexpr ll MOD = 0;//2024948111;
ld dotorad(ld K){return PI * K / 180.0;}
ld radtodo(ld K){return K * 180.0 / PI;}
mt19937 mt;
void randinit(){srand((unsigned)time(NULL));mt = mt19937(rand());}
int main(){
    ll N,D;
    cin >> N >> D;
    vector<ll> P(N),Q(N);
    for(ll i = 0;i < N;i++) cin >> P[i] >> Q[i];
    vector<tuple<ll,ll>> H(N);
    for(ll i = 0;i < N;i++) H[i] = make_tuple(-P[i] + Q[i],P[i]);
    sort(H.begin(),H.end());
    reverse(H.begin(),H.end());
    ll l = -MAX,r = 1;
    while(r - l > 1){
        ll m = (l + r) / 2,n = 0,ok = 1;
        for(ll i = 0;i < D;i++){
            ll o = 0;
            for(ll j = 0;j < N;j++){
                if(n - get<1>(H[i]) >= m){
                    n += get<0>(H[i]);
                    o = 1;
                    break;
                }
            }
            if(!o){
                ok = 0;
                break;
            }
        }
        if(ok) l = m;
        else r = m;
    }
    cout << l << endl;
}