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

#define all(v) (v).begin(),(v).end()
#define pb emplace_back
#define rep(i, n) for(int i=0;i<(n);i++)
#define foa(e, v) for(auto& e : v)
#define dout(a) cout<<fixed<<setprecision(10)<<a<<'\n';
#define Cout(a) cout<<a<<'\n';

using ll = long long;
using ld = long double;
using Int = __int128;
template <class T> using pqr = priority_queue<T, vector<T>, greater<T>>;

template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) {
    bool compare = a < b;
    if(compare) a = b;
    return compare;
}
template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) {
    bool compare = a > b;
    if(compare) a = b;
    return compare;
}
template <typename T> inline T back(std::set<T> &s) {
    return *s.rbegin();
}
template <typename T> inline T back(std::multiset<T> &s) {
    return *s.rbegin();
}
template <typename T> inline T pop_back(std::set<T> &s) {
    auto it = prev(s.end());
    T val = *it;
    s.erase(it); 
    return val;
}
template <typename T> inline T pop_back(std::multiset<T> &s) {
    auto it = prev(s.end());
    T val = *it;
    s.erase(it); 
    return val;
}

const int dy[8] = {-1, 0, 0, 1, 1, -1, 1, -1};
const int dx[8] = {0, -1, 1, 0, -1, -1, 1, 1};

const ll MOD7 = 1000000007, MOD998 = 998244353, INF = (3LL << 59);
const int inf = 1 << 30;
const char br = '\n';

void solve() {
    ll n, d; cin >> n >> d;
    map<ll, ll> mp;
    ll ans = 0;
    rep(i, n) {
        ll a, b; cin >> a >> b;
        mp[b] += a;
        chmax(ans, b);
    }
    vector<ll> a, b;
    for(auto [x, y] : mp) {
        a.pb(x);
        b.pb(y);
    }

    
    {
        ll sum = 0;
        ll base = d;
        n = a.size();
        for(int i = n - 1; i >= 0; i --) {
            sum += b[i];
            if(sum >= d) {
                base = a[i];
                break;
            }
        }
        priority_queue<pair<ll, ll>> pq;
        rep(i, n) pq.push({a[i], b[i]});

        sum = 0;
        while(1) {
            auto [x, y] = pq.top();
            pq.pop();
            if(x <= 0) {
                chmin(ans, sum);
                break;
            }
            chmin(ans, x + sum);
            if(x > base + d * 5) {
                ll num = max(1LL, (x - (base + d * 5)) / d);
                sum += num * y;
                if(sum >= ans) break;
                x -= num * d;
                pq.push({x, y});
            } else if(x < base - d * 5) break;
            else {
                sum += y;
                if(sum >= ans) break;
                x -= d;
                pq.push({x, y});
            }
        }
    }

    
    cout << ans << endl;
    
    
}

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    int testcase = 1; 
    // cin >> testcase;
    while(testcase --) solve();

    return 0;
}