#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const ll INF = 1LL << 60;
void chmin(ll &a, ll b) { a = min(a, b); }
// [0] * m + [1] * m + ... + [w-1] * m + [w] * inf ã®å…ˆé num é …ã®å’Œ
int f(int m, int w, int num) {
int k = min(num / m, w);
return k * (k - 1) / 2 * m + k * (num - k * m);
}
int main() {
int n, k;
cin >> n >> k;
vector<int> h(n), c(n), hs;
for (int i = 0; i < n; i++) {
cin >> h[i] >> c[i];
hs.push_back(h[i]);
hs.push_back(h[i] + 1);
}
sort(hs.begin(), hs.end());
hs.erase(unique(hs.begin(), hs.end()), hs.end());
int sz = hs.size();
vector<int> cnt(sz);
vector<ll> min_c(sz, INF);
for (int i = 0; i < n; i++) {
int p = lower_bound(hs.begin(), hs.end(), h[i]) - hs.begin();
++cnt[p];
chmin(min_c[p], c[i]);
}
vector dp(sz + 1, vector(n + 1, vector<ll>(n + 1, INF)));
dp[0][1][0] = 0;
ll mn = INF;
for (int i = 0; i < sz; i++) {
int w = (i + 1 < sz ? min(n, hs[i + 1] - hs[i]) : n);
for (int m = 1; m <= n; m++) {
for (int r = 0; r <= n; r++) {
ll now = dp[i][m][r];
if (now == INF) continue;
if (m < n) chmin(dp[i][m + 1][r], now + mn);
int num = r + cnt[i];
chmin(dp[i + 1][m][max(num - m * w, 0)], now + (ll) k * f(m, w, num));
}
}
chmin(mn, min_c[i]);
}
ll ans = INF;
for (int m = 1; m <= n; m++) chmin(ans, dp[sz][m][0]);
cout << ans << endl;
}
