//#pragma GCC optimize("Ofast")
//#pragma GCC target("avx,avx2,sse,sse2,sse3,ssse3,sse4,abm,popcnt,mmx")
#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef double db;
typedef long double ldb;
typedef complex<double> cd;

constexpr ll INF64 = 9'000'000'000'000'000'000, INF32 = 2'000'000'000, MOD = 1'000'000'007;
constexpr db PI = acos(-1);
constexpr bool IS_FILE = false, IS_TEST_CASES = false;

random_device rd;
mt19937 rnd32(rd());
mt19937_64 rnd64(rd());

template<typename T>
bool assign_max(T& a, T b) {
	if (b > a) {
		a = b;
		return true;
	}
	return false;
}

template<typename T>
bool assign_min(T& a, T b) {
	if (b < a) {
		a = b;
		return true;
	}
	return false;
}

template<typename T>
T square(T a) {
	return a * a;
}

template<>
struct std::hash<pair<ll, ll>> {
	ll operator() (pair<ll, ll> p) const {
		return ((__int128)p.first * MOD + p.second) % INF64;
	}
};

void solve() {
	ll n, k;
	cin >> n >> k;
	map<ll, vector<ll>> all;
	for (ll i = 0; i < n; i++) {
		ll x, y;
		cin >> x >> y;
		all[x].push_back(y);
	}
	ll x = all.begin()->first;
	sort(all[x].begin(), all[x].end());
	ll cost = all[x][0];
	ll ans = (all[x].size() - 1) * k;
	for (ll i = 1; i < all[x].size(); i++) {
		all[x + 1].push_back(all[x][i]);
	}
	all.erase(x);
	ll nc = 0;
	vector<ll> aa;
	x++;
	while (nc >= 0 || all.lower_bound(x) != all.end()) {
		aa.push_back(x);
		nc += all[x].size();
		if (nc < 0) {
			ll y = all.lower_bound(x)->first;
			nc = 0;
			if (x == y) {
				y++;
			}
			x = y;
		} else {
			x++;
		}
		nc--;
	}
	aa.push_back(x);
	vector<vector<vector<ll>>> dp(aa.size(), vector<vector<ll>>(n + 1, vector<ll>(n + 1, INF64)));
	dp[0][0][1] = ans;
	ll mcp = 0, msz = 1;
	for (ll i = 0; i + 1 < aa.size(); i++) {
		sort(all[aa[i]].begin(), all[aa[i]].end());
		for (ll cp = 0; cp <= mcp; cp++) {
			for (ll sz = 1; sz <= msz; sz++) {
				if (dp[i][cp][sz] == INF64) {
					continue;
				}
				ll ce = cp + all[aa[i]].size() - sz;
				for (ll addsz = 0; addsz <= max(0ll, ce); addsz++) {
					assign_min(dp[i + 1][max(0ll, ce - addsz)][sz + addsz], dp[i][cp][sz] + cost * addsz + max(0ll, ce - addsz) * k);
				}
			}
		}
		if (!all[aa[i]].empty()) {
			assign_min(cost, all[aa[i]][0]);
		}
		msz += all[aa[i]].size();
		mcp += all[aa[i]].size();
		mcp--;
		assign_max(mcp, 0ll);
	}
	ans = INF64;
	for (ll i = 0; i <= n; i++) {
		assign_min(ans, dp.back()[0][i]);
	}
	cout << ans << '\n';
}

int main() {
	if (IS_FILE) {
		freopen("", "r", stdin);
		freopen("", "w", stdout);
	}
    	ios_base::sync_with_stdio(false);
    	cin.tie(0);
    	cout.tie(0);
	ll t = 1;
	if (IS_TEST_CASES) {
		cin >> t;
	}
	for (ll i = 0; i < t; i++) {
		solve();
	}
}