#ifndef LOCAL
#pragma GCC Optimize("O3,Ofast,unroll-loops")
#pragma GCC Target("bmi,bmi2,avx,avx2")
#endif
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
#define int ll
#define f first
#define s second
#define mp make_pair
#define pb push_back
#define pii pair<int, int>
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin() (x).rend()
#define relax(x, y) (x) = min((x), (y));
#ifndef LOCAL
#define endl "\n"
#endif
mt19937 rnd(11);
void solve() {
int n, k;
cin >> n >> k;
vector<pii> a(n);
int mn = 1e9;
for (auto &x : a) {
cin >> x.f >> x.s;
mn = min(mn, x.f);
}
for (auto &x : a) {
x.f -= mn;
}
map<int, int> gmap;
for (auto &x : a) {
++gmap[x.f];
}
gmap[1e9 + n] = 0;
int temp = 0;
vector<int> cords;
int lst = 0;
for (auto &x : gmap) {
while (temp >= 0 && lst < x.f) {
cords.pb(lst++);
--temp;
}
if (x.s) {
cords.pb(x.f);
lst = x.f + 1;
temp += x.s - 1;
}
}
auto small = [&](int xx) {
return lower_bound(all(cords), xx) - cords.begin();
};
int H = cords.size();
vector<int> c(H, 1e18);
vector<int> g(H);
for (auto &x : a) {
++g[small(x.f)];
c[small(x.f)] = min(c[small(x.f)], x.s);
}
for (int i = 0; i < H - 1; ++i) {
c[i + 1] = min(c[i + 1], c[i]);
}
int dp[H][n + 1][n + 1];
for (int i = 0; i < H; ++i) {
for (int j = 0; j <= n; ++j) {
for (int k = 0; k <= n; ++k) {
dp[i][j][k] = 1e18;
}
}
}
dp[0][1][g[0] - 1] = (g[0] - 1) * k;
for (int i = 1; i < H; ++i) {
for (int q = 0; q <= n; ++q) {
for (int in = 0; in <= n - g[i] - q; ++in) {
if (dp[i - 1][q][in] == 1e18) {
continue;
}
for (int out = 0; out <= max(0ll, in + g[i] - q); ++out) {
int r = in + g[i] - out;
relax(dp[i][max(r, q)][out], dp[i - 1][q][in] + max(r - q, 0ll) * c[i - 1] + k * out * (cords[i] - cords[i - 1]));
}
}
}
}
int ans = 1e18;
for (int c = 0; c <= n; ++c) {
ans = min(ans, dp[H - 1][c][0]);
}
cout << ans << endl;
}
signed main() {
#ifdef LOCAL
freopen("in.txt", "r", stdin);
freopen("out.txt", "w", stdout);
#else
ios::sync_with_stdio(false);
cin.tie(0);
#endif
int t = 1;
// cin >> t;
while (t--) {
solve();
}
}
