#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define rep(i,n) for(int i=0;i<n;i++)
#define Pr pair<ll,ll>
#define Tp tuple<int,int,int>
#define all(v) v.begin(),v.end()
#define riano_ std::ios::sync_with_stdio(false);std::cin.tie(nullptr)
using Graph = vector<vector<ll>>;

const ll mod = 8e18;
template<uint64_t mod>
struct modint{
    uint64_t val;
    constexpr modint(const int64_t val_=0) noexcept:val((val_%int64_t(mod)+int64_t(mod))%int64_t(mod)){}
    constexpr modint operator-() const noexcept{
        return modint(*this)=mod-val;
    }
    constexpr modint operator+(const modint rhs) const noexcept{
        return modint(*this)+=rhs;
    }
    constexpr modint operator-(const modint rhs) const noexcept{
        return modint(*this)-=rhs;
    }
    constexpr modint operator*(const modint rhs) const noexcept{
        return modint(*this)*=rhs;
    }
    constexpr modint operator/(const modint rhs) const noexcept{
        return modint(*this)/=rhs;
    }
    constexpr modint &operator+=(const modint rhs) noexcept{
        val+=rhs.val;
        val-=((val>=mod)?mod:0);
        return (*this);
    }
    constexpr modint &operator-=(const modint rhs) noexcept{
        val+=((val<rhs.val)?mod:0);
        val-=rhs.val;
        return (*this);
    }
    constexpr modint &operator*=(const modint rhs) noexcept{
        val=val*rhs.val%mod;
        return (*this);
    }
    constexpr modint &operator/=(modint rhs) noexcept{
        uint64_t ex=mod-2;
        modint now=1;
        while(ex){
            now*=((ex&1)?rhs:1);
            rhs*=rhs,ex>>=1;
        }
        return (*this)*=now;
    }
    modint & operator++(){
        val++;
        if (val == mod) val = 0;
        return *this;
    }
    modint operator++(int){
        modint<mod> res = *this;
        ++*this;
        return res;
    }
    constexpr bool operator==(const modint rhs) noexcept{
        return val==rhs.val;
    }
    constexpr bool operator!=(const modint rhs) noexcept{
        return val!=rhs.val;
    }
    friend constexpr ostream &operator<<(ostream& os,const modint x) noexcept{
        return os<<(x.val);
    }
    friend constexpr istream &operator>>(istream& is,modint& x) noexcept{
        uint64_t t;
        is>>t,x=t;
        return is;
    }
};
typedef modint<mod> mint;

int main() {
    riano_; ll ans = -2e18;
    ll N,D; cin >> N >> D;
    ll p[N],q[N];
    vector<Pr> ps;
    rep(i,N){
        cin >> p[i] >> q[i];
        ps.push_back(make_pair(p[i],i));
    }
    sort(all(ps));
    rep(i,N){
        rep(j,N){
            if(i==j) continue;
            if(q[i]-p[i]+q[j]-p[j]<0) continue;
            ans = max(ans,min((-1)*p[i],q[i]-p[i]-p[j]));
        }
    }
    // if(ans!=-2e18){
    //     cout << ans << endl; return 0;
    // }
    bool ok = false;
    if(ans!=-2e18) ok = true;
    ll r = (D-1)%2;
    ll s = (D-1)/2;
    ll mp = ps[0].first; ll ip = ps[0].second;
    ll mp2 = ps[1].first; ll ip2 = ps[1].second;
    rep(i,N){
        rep(j,N){
            if(i==j) continue;
            if(q[i]-p[i]+q[j]-p[j]>0) continue;
            ll cand = 2e18;
            ll last = i; ll sec = j;
            if(r==0){
                last = j; sec = i;
            }
            ll sc = (q[i]-p[i]+q[j]-p[j])*s + (q[i]-p[i])*r;
            ll alt = ip2; ll rr = ip;
            if(last!=ip){
                cand = sc - mp;
            }
            else{
                cand = sc - mp2; alt = ip; rr = ip2;
            }
            ll c2 = sc - q[last]; 
            ll c3 = 2e18;
            if(D!=2){
                c3 = c2 +p[last]-q[sec];
            }
            cand = min({cand,c2,c3});
            if(ok) continue;
            ans = max(ans,cand);
            c2 += p[last];
            c2 -= p[alt];
            ll c1 = c2+q[alt]-p[rr];
            cand = max(cand,min({c1,c2,c3}));
            ans = max(ans,cand);
        }
    }
    cout << ans << endl;
}