#include <iostream>
#include <bits/stdc++.h>
#define ll long long
using namespace std;
const ll maxn = 2*1e5+5, INF = 4e15+9;
struct normalize{
vector<ll> poi, pot;
void add(ll x){
poi.push_back(x);
}
void start(){
sort(poi.begin(), poi.end());
if(poi.size() > 0) pot.push_back(poi[0]);
for(int i = 1; i < (int)poi.size(); i++){
if(poi[i] != poi[i-1]){
pot.push_back(poi[i]);
}
}
}
int encode(ll x){
return lower_bound(pot.begin(), pot.end(), x) - pot.begin()+1;
}
};
void solve(){
ll n, K;
cin >> n >> K;
vector<ll> h(n+1), c(n+1);
for(int i = 1; i <= n; i++){
cin >> h[i] >> c[i];
}
normalize norm;
for(int i = 1; i <= n; i++){
norm.add(h[i]);
norm.add(h[i]+1);
}
norm.start();
int N = norm.pot.size();
vector<ll> decode(N+2);
for(int i = 1; i <= N; i++){
decode[i] = norm.pot[i-1];
}
decode[N+1] = 1e10;
for(int i = 1; i <= n; i++){
h[i] = norm.encode(h[i]);
}
vector<ll> d(N+2, 0), mn(N+2, INF);
for(int i = 1; i <= n; i++){
d[h[i]]++;
mn[h[i]] = min(mn[h[i]], c[i]);
}
for(int i = 1; i <= N; i++){
mn[i] = min(mn[i], mn[i-1]);
}
auto pfmn = [&](int i) -> ll{
if(i == -1) return INF;
return mn[i];
};
auto chmin = [&](ll &a, ll b) -> void{
a = min(a, b);
};
vector<vector<ll>> f(n+1, vector<ll>(n+1, INF)), prv = f;
f[1][d[1]] = 0;
for(int i = 1; i < N+1; i++){
prv = f;
for(int j = 0; j <= n; j++){
for(int k = 0; k <= n; k++){
f[j][k] = INF;
}
}
for(int j = 1; j <= n; j++){
for(int k = 0; k <= n; k++){
//prv[j][k] -> f[j][d[i+1] + k-j] + (k-j) * K (subtask 5)
ll c = max(0, k - j); // amount that will get push
ll w = decode[i+1] - decode[i];
//if the rectangle w*j is enough to store all pushy balls
if(c <= (w-1) * j){
ll p = c/j, q = c%j;
ll whole_part = p*(p+1)/2 * j * K;
ll incomplete_part = q*(p+1)*K;
chmin(f[j][d[i+1]], prv[j][k] + whole_part + incomplete_part);
}else{
ll new_balls = c - (w-1)*j; // minus the balls to fill in the rectangle
ll rectangle_cost = w*(w-1)/2 * j * K; // (w-1 length)
ll extra_cost = new_balls * w * K;
if(d[i+1] + new_balls <= n) // just for overflow checking
chmin(f[j][d[i+1] + new_balls], prv[j][k] + rectangle_cost + extra_cost);
}
}
}
for(int j = 0; j < n; j++){
for(int k = 0; k <= n; k++){
//f[j][k] -> f[j+1][k] + pfmn(i)
chmin(f[j+1][k], f[j][k] + pfmn(i));
}
}
}
ll ans = INF;
for(int j = 0; j <= n; j++){
ans = min(ans, f[j][0]);
}
cout << ans;
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
solve();
}
