#define _CRT_SECURE_NO_WARNINGS
#include "bits/stdc++.h"
#include <iostream>
#include <random>
#include <unordered_map>
#include <unordered_set>
#include <immintrin.h>
#ifdef _MSC_VER
#include <ppl.h>
#endif

using namespace std;

//呪文
#define DUMPOUT std::cerr
#define dump(...) DUMPOUT<<"  ";DUMPOUT<<#__VA_ARGS__<<" :["<<__LINE__<<":"<<__FUNCTION__<<"]"<<std::endl;DUMPOUT<<"    ";dump_func(__VA_ARGS__)

typedef unsigned uint; typedef long long ll; typedef unsigned long long ull; typedef pair<int, int> pii; typedef pair<ll, ll> pll; typedef pair<double, double> pdd; typedef pair<string, string> pss;
template <typename _KTy, typename _Ty> ostream& operator << (ostream& o, const pair<_KTy, _Ty>& m) { o << "{" << m.first << ", " << m.second << "}"; return o; }
template <typename _KTy, typename _Ty> ostream& operator << (ostream& o, const map<_KTy, _Ty>& m) { if(m.empty()) { o << "{ }"; return o; } o << "{" << *m.begin(); for(auto itr = ++m.begin(); itr != m.end(); itr++) { o << ", " << *itr; } o << "}"; return o; }
template <typename _KTy, typename _Ty> ostream& operator << (ostream& o, const unordered_map<_KTy, _Ty>& m) { if(m.empty()) { o << "{ }"; return o; } o << "{" << *m.begin(); for(auto itr = ++m.begin(); itr != m.end(); itr++) { o << ", " << *itr; } o << "}"; return o; }
template <typename _Ty> ostream& operator << (ostream& o, const vector<_Ty>& v) { if(v.empty()) { o << "{ }"; return o; } o << "{" << v.front(); for(auto itr = ++v.begin(); itr != v.end(); itr++) { o << ", " << *itr; } o << "}"; return o; }
template <typename _Ty> ostream& operator << (ostream& o, const set<_Ty>& s) { if(s.empty()) { o << "{ }"; return o; } o << "{" << *(s.begin()); for(auto itr = ++s.begin(); itr != s.end(); itr++) { o << ", " << *itr; } o << "}"; return o; }
template <typename _Ty> ostream& operator << (ostream& o, const unordered_set<_Ty>& s) { if(s.empty()) { o << "{ }"; return o; } o << "{" << *(s.begin()); for(auto itr = ++s.begin(); itr != s.end(); itr++) { o << ", " << *itr; }	o << "}"; return o; }
template <typename _Ty> ostream& operator << (ostream& o, const stack<_Ty>& s) { if(s.empty()) { o << "{ }"; return o; } stack<_Ty> t(s); o << "{" << t.top(); t.pop(); while(!t.empty()) { o << ", " << t.top(); t.pop(); } o << "}";	return o; }
template <typename _Ty> ostream& operator << (ostream& o, const list<_Ty>& l) { if(l.empty()) { o << "{ }"; return o; } o << "{" << l.front(); for(auto itr = ++l.begin(); itr != l.end(); ++itr) { o << ", " << *itr; } o << "}"; return o; }
template <typename _KTy, typename _Ty> istream& operator >> (istream& is, pair<_KTy, _Ty>& m) { is >> m.first >> m.second; return is; }
template <typename _Ty> istream& operator >> (istream& is, vector<_Ty>& v) { for(size_t i = 0; i < v.size(); i++) is >> v[i]; return is; }
namespace aux { // print tuple
  template<typename Ty, unsigned N, unsigned L> struct tp { static void print(ostream& os, const Ty& v) { os << get<N>(v) << ", "; tp<Ty, N + 1, L>::print(os, v); } };
  template<typename Ty, unsigned N> struct tp<Ty, N, N> { static void print(ostream& os, const Ty& v) { os << get<N>(v); } };
}
template<typename... Tys> ostream& operator<<(ostream& os, const tuple<Tys...>& t) { os << "{"; aux::tp<tuple<Tys...>, 0, sizeof...(Tys) - 1>::print(os, t); os << "}"; return os; }

template<typename A, size_t N, typename T> inline void Fill(A(&array)[N], const T &val) { std::fill((T*)array, (T*)(array + N), val); }

void dump_func() { DUMPOUT << std::endl; }
template <class Head, class... Tail> void dump_func(Head&& head, Tail&&... tail) { DUMPOUT << head; if(sizeof...(Tail) == 0) { DUMPOUT << " "; } else { DUMPOUT << ", "; } dump_func(std::move(tail)...); }

template<typename Ty>Ty modpow(Ty n, Ty e, Ty m) { Ty result = 1; while(e > 0) { if(e & 1)result = (result*n) % m; e >>= 1; n = (n*n) % m; }return result; }


#define PI 3.14159265358979323846
#define EPS 1e-10
#define FOR(i,a,n) for(int i=(a);i<(n);++i)
#define REP(i,n)  FOR(i,0,n)
#define all(j) (j).begin(), (j).end()
#define SZ(j) ((int)(j).size())
#define fake false



int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);

  ll N, M;
  cin >> N >> M;
  vector<pll> p;
  REP(i, N){
    ll a, b;
    cin >> a >> b;
    if(a > M) continue;
    p.emplace_back(a, b);
  }

  if(p.empty()){
    cout << 0 << endl;
    return 0;
  }

  sort(all(p), [&](const pll& a, const pll& b){
    return a.first == b.first ? a.second > b.second : a.first > b.first;
  });

  map<ll, ll> mp;
  int pos = 0;
  for(ll day = M; day >= 1; day--){
    mp[0]++; // add slot
    for(; pos < p.size() && p[pos].first == day; pos++){
      ll minb = mp.begin()->first;
      if(minb < p[pos].second){
        mp[minb]--;
        if(mp[minb] == 0) mp.erase(minb);
        mp[p[pos].second]++;
      }
    }
  }

  ll ans = 0;
  for(auto e : mp) ans += e.first * e.second;

  cout << ans << endl;

  return 0;
}

Measured memory_bytes: 2420