#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); ++i)

using namespace std;
using ll = long long;
using P = pair<ll, ll>;

int main() {
    int n, d;
    cin >> n >> d;
    
    vector<ll> a(n), b(n);
    rep(i, n) cin >> a[i] >> b[i];
    
    vector<int> is(n);
    rep(i, n) is[i] = i;
    
    ll sum = 0;
    rep(i, n) sum += a[i];
    
    ll t = 0;
    if (sum >= d) {
        sort(is.begin(), is.end(), [&](int i, int j) {
            return b[i] < b[j];
        });
        
        ll suf = sum;
        for (int i = 0; i < n;) {
            int j = i;
            while (j < n and b[is[j]] == b[is[i]]) {
                suf -= a[is[j]];
                ++j;
            }
            
            if (suf < d) {
                t = b[is[i]];
                break;
            }
            
            i = j;
        }
    }
    
    ll now = t;
    rep(i, n) {
        if (b[i] > t) {
            now += ll(b[i]-t+d-1)/d*a[i];
        }
    }
    
    ll ans = now;
    vector<P> events;
    rep(i, n) {
        ll x = t + (b[i]%d-t%d+d)%d;
        if (t < x and x <= b[i]) {
            events.emplace_back(x, a[i]);
        }
    }
    sort(events.begin(), events.end());
    
    ll pos = t;
    for (int i = 0; i < events.size();) {
        ll x = events[i].first;
        ll w = 0;
        while (i < events.size() and events[i].first == x) {
            w += events[i].second;
            ++i;
        }
        now += x-pos-w;
        pos = x;
        ans = min(ans, now);
    }
    
    cout << ans << '\n';
    
    return 0;
}