#include <bits/stdc++.h>
using namespace std;
using ll=long long;
const ll ILL=2167167167167167167;
const int INF=2100000000;
#define rep(i,a,b) for (int i=(int)(a);i<(int)(b);i++)
#define all(p) p.begin(),p.end()
template<class T> using pq_ = priority_queue<T, vector<T>, greater<T>>;
template<class T> int LB(vector<T> &v,T a){return lower_bound(v.begin(),v.end(),a)-v.begin();}
template<class T> int UB(vector<T> &v,T a){return upper_bound(v.begin(),v.end(),a)-v.begin();}
template<class T> bool chmin(T &a,T b){if(b<a){a=b;return 1;}else return 0;}
template<class T> bool chmax(T &a,T b){if(a<b){a=b;return 1;}else return 0;}
template<class T> void So(vector<T> &v) {sort(v.begin(),v.end());}
template<class T> void Sore(vector<T> &v) {sort(v.begin(),v.end(),[](T x,T y){return x>y;});}
bool yneos(bool a,bool upp=false){if(a){cout<<(upp?"YES\n":"Yes\n");}else{cout<<(upp?"NO\n":"No\n");}return a;}
template<class T> void vec_out(vector<T> &p,int ty=0){
    if(ty==2){cout<<'{';for(int i=0;i<(int)p.size();i++){if(i){cout<<",";}cout<<'"'<<p[i]<<'"';}cout<<"}\n";}
    else{if(ty==1){cout<<p.size()<<"\n";}for(int i=0;i<(int)(p.size());i++){if(i) cout<<" ";cout<<p[i];}cout<<"\n";}}
template<class T> T vec_min(vector<T> &a){assert(!a.empty());T ans=a[0];for(auto &x:a) chmin(ans,x);return ans;}
template<class T> T vec_max(vector<T> &a){assert(!a.empty());T ans=a[0];for(auto &x:a) chmax(ans,x);return ans;}
template<class T> T vec_sum(vector<T> &a){T ans=T(0);for(auto &x:a) ans+=x;return ans;}
int pop_count(long long a){int res=0;while(a){res+=(int)(a&1),a>>=1;}return res;}
template<class T> T square(T a){return a * a;}



void solve();
// DEAR MYSTERIES / TOMOO
int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int t = 1;
    // cin >> t;
    rep(i, 0, t) solve();
}

void solve(){
    ll N, D;
    cin >> N >> D;
    struct F {
        ll c;
        ll h;
    };
    vector<F> p(N);
    rep(i, 0, N) cin >> p[i].c >> p[i].h;
    sort(all(p), [&](F l, F r) {
        return l.h > r.h;
    });
    p.push_back({INF, 0});
    int ind = 0;
    ll sum = 0;
    while (p[ind].c + sum < D) {
        sum += p[ind].c;
        ind++;
    }
    ll ans = p[ind].h;
    p.resize(ind);
    {
        vector<pair<ll, ll>> q;
        ll del = ans;
        for (auto [c, h] : p) {
            h -= del;
            if (h) {
                ans += c * ((h + D - 1) / D);
                if (h % D) q.push_back({h % D, c});
            }
        }
        So(q);
        ll tmp = 0;
        ll c_sum = 0;
        for (auto [h, c] : q) {
            c_sum += c;
            chmin(tmp, h - c_sum);
        }
        ans += tmp;
    }
    cout << ans << "\n";
}

// https://jag-icpc.org/?plugin=attach&refer=2026%2FPractice%2F夏合宿%2F講評&openfile=day3_F.pdf