#include <bits/stdc++.h>
//#include "includeall.h"
//#include <ext/pb_ds/assoc_container.hpp>
//#include <ext/pb_ds/tree_policy.hpp>
#define endl '\n'
#define f first
#define s second
#define pb push_back
#define mp make_pair
#define lb lower_bound
#define ub upper_bound
#define input(x) scanf("%lld", &x);
#define input2(x, y) scanf("%lld%lld", &x, &y);
#define input3(x, y, z) scanf("%lld%lld%lld", &x, &y, &z);
#define input4(x, y, z, a) scanf("%lld%lld%lld%lld", &x, &y, &z, &a);
#define print(x, y) printf("%lld%c", x, y);
#define show(x) cerr << #x << " is " << x << endl;
#define show2(x,y) cerr << #x << " is " << x << " " << #y << " is " << y << endl;
#define show3(x,y,z) cerr << #x << " is " << x << " " << #y << " is " << y << " " << #z << " is " << z << endl;
#define all(x) x.begin(), x.end()
#define discretize(x) sort(x.begin(), x.end()); x.erase(unique(x.begin(), x.end()), x.end());
#define FOR(i, x, n) for (ll i =x; i<=n; ++i)
#define RFOR(i, x, n) for (ll i =x; i>=n; --i)
#pragma GCC optimize("O3","unroll-loops")
using namespace std;
mt19937_64 rnd(chrono::steady_clock::now().time_since_epoch().count());
//using namespace __gnu_pbds;
//#define ordered_set tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update>
//#define ordered_multiset tree<int, null_type, less_equal<int>, rb_tree_tag, tree_order_statistics_node_update>
typedef long long ll;
typedef long double ld;
typedef pair<ld, ll> pd;
typedef pair<string, ll> psl;
typedef pair<ll, ll> pi;
typedef pair<ll, pi> pii;
typedef pair<pi, pi> piii;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
ll dp[605][305][305];
ll cal(ll n, ll k, ll gap)
{
ll g = min((n/k), gap);
return (g)*(g-1)/2*k + g*(n - k * g);
}
int main()
{
ll n, ck;
cin >> n >> ck;
vector<ll> dis;
vector<pi> a(n);
for (ll i=0; i<n; ++i) {cin >> a[i].f >> a[i].s; dis.pb(a[i].f); dis.pb(a[i].f+1);}
discretize(dis);
ll nn = dis.size();
vector<ll> min_cost(nn, ll(1e15)), cnt(nn, 0);
for (ll i=0; i<n; ++i)
{
a[i].f = lb(all(dis), a[i].f)-dis.begin();
cnt[a[i].f]++;
min_cost[a[i].f] = min(min_cost[a[i].f], a[i].s);
}
for (ll i=0; i<=nn; ++i)for (ll j=0; j<=n; ++j)for (ll k=0; k<=n; ++k) dp[i][j][k] = ll(1e15);
dp[0][1][0] = 0;
ll ps = ll(1e15);
for (ll i=0; i<nn; ++i)
{
ll gap = ((i==nn-1)?n:dis[i+1] - dis[i]);
for (ll j=1; j<=n; ++j)
{
for (ll k=0; k<=n; ++k)
{
// buy more connection
if (j + 1 <= n) dp[i][j + 1][k] = min(dp[i][j + 1][k], dp[i][j][k] + ps);
// just connect to the next one
dp[i+1][j][max(0LL, cnt[i] + k - j*gap)] = min(dp[i+1][j][max(0LL, cnt[i] + k - j*gap)], dp[i][j][k] + ck * cal(k + cnt[i], j, gap));
//show(dp[i][j][k]);
}
}
ps = min(ps, min_cost[i]);
}
ll ans = ll(1e15);
for (ll i=1; i<=n; ++i) { ans = min(ans, dp[nn][i][0]);}
cout << ans << endl;
return 0;
}
