#include "bits/stdc++.h"
using namespace std;
#ifdef Nero
#include "Deb.h"
#else
#define debug(...)
#endif
const int N = 42;
const long long INF = 1e15;
int n, k;
int h[N], c[N];
int cnt[N * 2];
int prefsum[N * 2];
int prefcnt[N * 2];
int pref[N * 2 + 1];//mnC
long long dp[N * 2 + 1][N][N][N];
void init() {
prefcnt[0] = cnt[0];
for (int i = 1; i < N * 2; ++i) {
prefcnt[i] = cnt[i] + prefcnt[i - 1];
prefsum[i] += prefsum[i - 1];
}
for (int i = 1; i < N * 2; ++i) {
if (pref[i - 1] == 0) continue;
if (pref[i] == 0 || (c[pref[i]] > c[pref[i - 1]])) {
pref[i] = pref[i - 1];
}
}
for (int height = 0; height < N * 2; ++height) {
for (int available = 0; available < n; ++available) {
for (int carry = 0; carry < n; ++carry) {
for (int mn = 0; mn <= n; ++mn) {
dp[height][available][carry][mn] = INF;
}
}
}
}
for (int i = 1; i <= n; ++i) {
dp[h[i] + 1][1][prefcnt[h[i]] - 1][i] = ((prefcnt[h[i]] - 1) * (h[i] + 1) - (prefsum[h[i]] - h[i])) * k;
}
}
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
cin >> n >> k;
for (int i = 1; i <= n; ++i) {
cin >> h[i] >> c[i];
cnt[h[i]]++;
prefsum[h[i]] += h[i];
if (pref[h[i]] == 0 || c[pref[h[i]]] > c[i]) {
pref[h[i]] = i;
}
}
init();
for (int height = 0; height < N * 2; ++height) {
for (int available = 0; available < n; ++available) {
for (int carry = 0; carry < n; ++carry) {
for (int mn = 1; mn <= n; ++mn) {
int t = carry + cnt[height];
for (int i = 0; i <= max(0, t - available); ++i) {
int ncarry = carry - i + cnt[height];
int navailable = available + i;
int nmn = (i == 0 ? mn : pref[height]);
if (available && ncarry) {
int tmp = min(ncarry, available);
ncarry -= tmp;
nmn = pref[height];
}
long long cost = ncarry * k + i * c[mn];
dp[height + 1][navailable][ncarry][nmn] = min(dp[height + 1][navailable][ncarry][nmn], dp[height][available][carry][mn] + cost);
}
}
}
}
}
long long ans = LLONG_MAX;
for (int available = 1; available < n; ++available) {
ans = min(ans, dp[N * 2 - 1][available][0][pref[N * 2 - 1]]);
}
cout << ans << '\n';
return 0;
}
