#include <algorithm>
#include <iostream>
#include <numeric>
#include <cassert>
#include <vector>
typedef long long llong;
const int MAXN = 40 + 10;
const int MAXH = 40 + 10;
const int MAXB = 1024;
const llong INF = 1e18;
int n;
llong k;
llong h[MAXN];
llong c[MAXN];
llong dp[2 * MAXH][MAXN][MAXN][MAXN];
bool bl[2 * MAXH][MAXN][MAXN][MAXN];
int cntWith[2 * MAXH];
int minC[2 * MAXH];
llong maxH;
llong f(int height, int waiting, int free, int min)
{
if (height == 2 * MAXH)
{
if (waiting > 0) return INF;
return 0;
}
if (bl[height][waiting][free][min])
{
return dp[height][waiting][free][min];
}
bl[height][waiting][free][min] = true;
dp[height][waiting][free][min] = f(height + 1, waiting + cntWith[height], free, min);
int newMin = min;
if (c[minC[height]] < c[min])
{
newMin = minC[height];
}
for (int choose = 1 ; choose <= waiting + cntWith[height] ; ++choose)
{
if (min == 0 && choose == 2) break;
dp[height][waiting][free][min] = std::min(dp[height][waiting][free][min], f(height + 1, waiting + cntWith[height] - choose, std::max(free, choose), newMin) + std::max(0, choose - free) * c[min] + k * choose * height - k * cntWith[height] * height);
}
return dp[height][waiting][free][min];
}
void solve()
{
c[0] = INF;
for (int i = 1 ; i <= n ; ++i)
{
cntWith[h[i]]++;
if (c[minC[h[i]]] > c[i])
{
minC[h[i]] = i;
}
}
std::cout << f(0, 0, 1, 0) << '\n';
}
void input()
{
std::cin >> n >> k;
for (int i = 1 ; i <= n ; ++i)
{
std::cin >> h[i] >> c[i];
maxH = std::max(maxH, h[i]);
}
}
void fastIOI()
{
std::ios_base :: sync_with_stdio(0);
std::cout.tie(nullptr);
std::cin.tie(nullptr);
}
int main()
{
fastIOI();
input();
solve();
return 0;
}
