#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> pll;
#define vc vector
#define st first
#define nd second
#define all(a) a.begin(), a.end()
#define sz(a) (ll)a.size()
#define pub push_back
#define pob pop_back
struct P {
ll h, c;
};
ll n, X;
vc<P> a;
void input() {
cin >> n >> X;
a.resize(n);
for (ll i = 0; i < n; i++)
cin >> a[i].h >> a[i].c;
}
void solve() {
sort(all(a), [](auto &p, auto &q) {
if (p.h != q.h)
return p.h < q.h;
return p.c < q.c;
});
ll ans = 0;
for (ll i = 1; i < n; i++)
if (a[i].h == a[0].h) {
a[i].h++;
ans += X;
}
ll bf = a[0].c;
ll mn = a[0].c;
deque<ll> fr;
fr.pub(a[0].h);
for (ll i = 1; i < n; i++) {
if (a[i].h != a[i - 1].h)
mn = min(mn, bf);
if (not fr.empty() and fr.front() < a[i].h)
fr.pop_front();
else
ans += mn;
fr.pub(a[i].h);
bf = min(bf, a[i].c);
}
cout << ans << "\n";
}
void program() {
input();
solve();
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
program();
return 0;
}
