// Hallelujah, praise the one who set me free
// Hallelujah, death has lost its grip on me
// You have broken every chain, There's salvation in your name
// Jesus Christ, my living hope
#include <bits/stdc++.h>
using namespace std;
#define REP(i, s, e) for (int i = (s); i < (e); i++)
#define RREP(i, s, e) for (int i = (s); i >= (e); i--)
template <class T>
inline bool mnto(T& a, T b) {return a > b ? a = b, 1 : 0;}
template <class T>
inline bool mxto(T& a, T b) {return a < b ? a = b, 1: 0;}
typedef unsigned long long ull;
typedef long long ll;
typedef long double ld;
#define FI first
#define SE second
typedef pair<int, int> ii;
typedef pair<ll, ll> pll;
typedef tuple<int, int, int> iii;
#define ALL(_a) _a.begin(), _a.end()
#define SZ(_a) (int) _a.size()
#define pb push_back
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<ii> vii;
typedef vector<iii> viii;
#ifndef DEBUG
#define cerr if (0) cerr
#endif
const int INF = 1000000005;
const ll LINF = 1000000000000000005ll;
const int MAXN = 305;
int n, m;
ii hc[MAXN];
int lst[MAXN], mnc[MAXN];
map<int, vi> mp;
map<int, int> id;
vi vh;
ll dp[MAXN][MAXN][MAXN];
ll solve() {
mp.clear();
id.clear();
vh.clear();
REP (i, 1, n + 1) {
mp[hc[i].FI].pb(hc[i].SE);
}
sort(hc + 1, hc + n + 1);
RREP (i, n, 1) {
if (hc[i + 1].FI == hc[i].FI) {
lst[i] = lst[i + 1];
} else {
lst[i] = i;
}
}
mnc[0] = INF;
REP (i, 1, n + 1) {
mnc[i] = min(mnc[i - 1], hc[i].SE);
}
REP (i, 1, n + 1) {
cerr << hc[i].FI << ' ' << hc[i].SE << ' ' << mnc[i] << '\n';
}
int rht = 0;
for (auto [h, v] : mp) {
mxto(rht, h);
rht += SZ(v);
for (int i = h; i < rht; i++) {
vh.pb(i);
}
}
vh.pb(-1);
sort(ALL(vh));
vh.erase(unique(ALL(vh)), vh.end());
REP (i, 1, SZ(vh)) {
id[vh[i]] = i;
}
REP (i, 0, n + 1) {
REP (j, 0, n + 1) {
REP (k, 0, SZ(vh)) {
dp[i][j][k] = LINF;
}
}
}
dp[1][1][1] = 0;
REP (i, 1, n + 1) {
REP (j, 1, i + 1) {
REP (k, 1, SZ(vh)) {
if (dp[i][j][k] == LINF) {
continue;
}
cerr << i << ' ' << j << ' ' << vh[k] << ": " << dp[i][j][k] << '\n';
int nk = max(k + 1, id[hc[i + 1].FI]);
ll w = 0;
REP (l, 1, j + 1) {
if (i + l > n || vh[nk] < hc[i + l].FI) {
continue;
}
w += (ll) (vh[nk] - hc[i + l].FI) * m;
mnto(dp[i + l][j][nk], dp[i][j][k] + w);
}
/*
int ni = min(i + j, lst[i + 1]),
nk = max(k + 1, id[hc[i + 1].FI]);
mnto(dp[i + j][j][nk], dp[i][j][k] + (ni - i) *
(vh[nk] - hc[i + 1].FI) * m);
*/
if (i > 1) {
mnto(dp[i + 1][j + 1][k], dp[i][j][k] + mnc[i - j] +
(ll) max(0, vh[k] - hc[i + 1].FI) * m);
}
}
}
}
ll ans = LINF;
REP (j, 1, n + 1) {
REP (k, 1, SZ(vh)) {
mnto(ans, dp[n][j][k]);
}
}
return ans;
}
int main() {
#ifndef DEBUG
ios::sync_with_stdio(0), cin.tie(0);
#endif
cin >> n >> m;
REP (i, 1, n + 1) {
cin >> hc[i].FI >> hc[i].SE;
}
sort(hc + 1, hc + n + 1);
lst[n + 1] = n + 1;
hc[n + 1].FI = INF;
ll ans = LINF;
ll res = 0;
REP (i, 1, n + 1) {
REP (j, 1, i) {
res += (ll) (hc[i].FI - hc[j].FI) * m;
hc[j].FI = hc[i].FI;
}
mnto(ans, res + solve());
}
cout << ans << '\n';
return 0;
}
