#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
// using namespace atcoder;
// using mint = modint1000000007;
// const int mod = 1000000007;
// using mint = modint998244353;
// const int mod = 998244353;
// const int INF = 1e9;
// const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i, l, r) for (int i = (l); i < (r); ++i)
#define rrep(i, n) for (int i = (n)-1; i >= 0; --i)
#define rrep2(i, l, r) for (int i = (r)-1; i >= (l); --i)
#define all(x) (x).begin(), (x).end()
#define allR(x) (x).rbegin(), (x).rend()
#define P pair<int, int>
template<typename A, typename B> inline bool chmax(A& a, const B& b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A& a, const B& b) { if (a > b) { a = b; return true; } return false; }

int main() {
	std::ios::sync_with_stdio(false);
	std::cin.tie(nullptr);
	int n, d; cin >> n >> d;
	vector<P>v(n);// 体力/人数
	long long sum = 0;
	rep(i, n) {
		cin >> v[i].second >> v[i].first;
		sum += v[i].second;
	}
	sort(allR(v));
	int k = min<long long>(d, sum);
	long long base = 0, cur = 0;
	map<long long, long long>eve;
	for (auto [b, a] : v) {
		long long tmp = min(k, a);
		if (tmp == 0)break;
		base += tmp * (b / d);
		if (b % d) {
			eve[b % d] += tmp;
			cur += tmp;
		}
		k -= tmp;
	}
	// 全体攻撃をどれだけするか
	long long add = cur;
	for (auto [r, cnt] : eve) {
		cur -= cnt;
		chmin(add, cur + r);
	}
	cout << base + add << endl;
	return 0;
}