#include<bits/stdc++.h>
using namespace std;
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,ans = -MAX;
    cin >> N >> D;
    vector<ll> P(N),Q(N);
    for(ll i = 0;i < N;i++) cin >> P[i] >> Q[i];
    for(ll i = 0;i < N;i++) for(ll j = 0;j < N;j++) if(i != j){
        ll m = min(-P[i],Q[i] - P[i] - P[j]);
        ll s = Q[i] + Q[j] - P[i] - P[j];
        if(s >= 0){
            ans = max(ans,m);
        }
        else{
            if(D % 2 == 0){
                ll p = s * (D / 2 - 1);
                ans = max(ans,p + m);
            }
            else{
                ll p = s * (D / 2 - 1);
                ans = max(ans,min(p + m,p + s - P[i]));
            }
        }
    }
    cout << ans << endl;
}