// #pragma GCC optimize("O3")
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,tune=native")
#include <bits/stdc++.h>
using namespace std;

#define PB emplace_back
#define int long long
#define ll long long
#define vi vector<int>
#define siz(a) ((int) ((a).size()))
#define rep(i, a, b) for (int i = (a); i <= (b); ++i)
#define per(i, a, b) for (int i = (a); i >= (b); --i)
void print(vi n) { rep(i, 0, siz(n) - 1) cerr << n[i] << " \n"[i == siz(n) - 1]; }

const int N = 300, inf = 1e15;
int a, b, f[N + 5][N + 5], g[N + 5][N + 5], p[N + 5];
struct pii {int x, y;} s[N + 5];

signed main() {
	// freopen(".in", "r", stdin);
	// freopen(".out", "w", stdout);
	ios::sync_with_stdio(0);
	cin.tie(0), cout.tie(0);
	cin >> a >> b;
	rep(i, 1, a) cin >> s[i].x >> s[i].y;
	sort(s + 1, s + a + 1, [&] (pii x, pii y) {
		return x.x < y.x || (x.x == y.x && x.y < y.y);
	});
	int now = s[1].x - 1, top = 0;
	rep(i, 1, a) {
		now = max(now + 1, s[i].x);
		p[++top] = now;
	}
	p[++top] = inf;

	top = unique(p + 1, p + top + 1) - p - 1;
	memset(f, 0x3f, sizeof(f));
	f[0][1] = 0;
	int l = 1, mn = inf;
	rep(i, 1, top) {
		int r = l - 1;
		for(; r + 1 <= a && s[r + 1].x == p[i]; ++r);
		memset(g, 0x3f, sizeof(g));
		rep(j, 0, a) rep(k, 1, a) if(f[j][k] < inf) {
			int x = j + r - l + 1;
			if(x <= k) g[0][k] = min(g[0][k], f[j][k]);
			else {
				rep(t, 0, x - k) {
					int y = x - k - t;
					g[y][t + k] = min(g[y][t + k], f[j][k] + mn * t + y * b * (p[i + 1] - p[i]));
				}
			}
		}
		rep(j, 0, a) rep(k, 1, a) f[j][k] = g[j][k];
		rep(j, l, r) mn = min(mn, s[j].y);
		l = r + 1;
	}
	int ans = inf;
	rep(i, 0, a) rep(j, 1, a) ans = min(ans, f[0][j]);
	cout << ans;
	return cerr << endl << 1.0 * clock() / CLOCKS_PER_SEC << endl, 0;
}