// In the name of Allah
#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef complex<ld> cld;
#define all(x) (x).begin(),(x).end()
#define len(x) ((ll) (x).size())
#define F first
#define S second
#define pb push_back
#define sep ' '
#define endl '\n'
#define Mp make_pair
#define kill(x) cout << x << '\n', exit(0)
#define set_dec(x) cout << fixed << setprecision(x);
#define file_io(x,y) freopen(x, "r", stdin); freopen(y, "w", stdout);
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int n; ll k;
const int maxn = 600 + 4;
const ll oo = 1e18 + 4;
pll A[maxn]; vector<ll> arr;
ll dp[2][maxn][maxn], mn[maxn], cnt[maxn];
int GI(ll x) {
return lower_bound(all(arr), x) - arr.begin();
}
int main() {
ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);
cin >> n >> k;
for (int i = 0; i < n; i++) cin >> A[i].F >> A[i].S;
sort(A, A + n); ll res = 0;
for (int i = 1; i < n; i++) {
if (A[i].F == A[0].F) {
A[i].F++; res += k;
}
}
sort(A, A + n);
ll x = 0;
for (int i = 0; i < n; i++) {
x = max(x, A[i].F);
arr.pb(x); x++;
}
fill(mn, mn + len(arr), oo);
for (int i = 0; i < n; i++) {
int j = GI(A[i].F);
mn[j] = min(mn[j], A[i].S);
cnt[j]++;
}
for (int i = 1; i < len(arr); i++) mn[i] = min(mn[i], mn[i - 1]);
for (int i = 0; i < len(arr); i++) {
if (i == 0) {
for (int j1 = 0; j1 <= n; j1++) {
for (int j2 = 0; j2 <= n; j2++) {
dp[i & 1][j1][j2] = oo;
if (j1 == 1 && j2 == 0) dp[i & 1][j1][j2] = 0;
}
}
}
else {
for (int j1 = 0; j1 <= n; j1++) {
for (int j2 = 1; j2 <= n; j2++) {
dp[i & 1][j1 + 1][j2 - 1] = min(dp[i & 1][j1 + 1][j2 - 1], dp[i & 1][j1][j2] + mn[i - 1]);
}
}
}
if (i + 1 >= len(arr)) continue;
for (int j1 = 0; j1 <= n; j1++) {
for (int j2 = 0; j2 <= n; j2++) {
dp[(i + 1) & 1][j1][j2] = oo;
}
}
for (int j1 = 0; j1 <= n; j1++) {
for (int j2 = 0; j2 <= n; j2++) {
if (dp[i & 1][j1][j2] == oo) continue;
ll x = dp[i & 1][j1][j2] + (k * (arr[i + 1] - arr[i]) * j2);
int r1 = j1, r2 = j2 + cnt[i + 1];
int e = min(r1, r2); r2 -= e;
dp[(i + 1) & 1][r1][r2] = min(dp[(i + 1) & 1][r1][r2], x);
}
}
}
ll ans = oo;
for (int j1 = 0; j1 <= n; j1++) ans = min(ans, dp[(len(arr) - 1) & 1][j1][0]);
ans += res;
cout << ans << endl;
return 0;
}
