#include<cassert>
#include <bits/stdc++.h>
#ifndef IO_HPP
#define IO_HPP
#include<iostream>
#include<vector>
#include<queue>
#include<stack>
#include<array>
#include<map>
#include<unordered_map>
#include<set>
#include<unordered_set>
using namespace std;
template<typename T1,typename T2>istream &operator>>(istream&,pair<T1,T2>&);
template<typename...Args>istream &operator>>(istream&,tuple<Args...>&a);
template<typename T>istream &operator>>(istream&is,vector<T>&a);
template<typename T,size_t N>istream &operator>>(istream&is,array<T,N>&a);
template<typename T1,typename T2>
istream &operator>>(istream&is,pair<T1,T2>&a){
  is>>a.first>>a.second;
  return is;
}
template<size_t pos,typename...Args>
void read_tuple(istream&is,tuple<Args...>&a){
  if constexpr(pos<tuple_size<tuple<Args...>>::value){
    is>>get<pos>(a);
    read_tuple<pos+1>(is,a);
  }
}
template<typename...Args>
istream &operator>>(istream&is,tuple<Args...>&a){
  read_tuple<0>(is,a);
  return is;
}
template<typename T>
istream &operator>>(istream&is,vector<T>&a){
  for(T&x:a)is>>x;
  return is;
}
template<typename T,size_t N>
istream &operator>>(istream&is,array<T,N>&a){
  for(T&x:a)is>>x;
  return is;
}
template<typename T1,typename T2>ostream &operator<<(ostream&os,const pair<T1,T2>&);
template<typename...Args>ostream &operator<<(ostream&os,const tuple<Args...>&);
template<typename T>ostream &operator<<(ostream&os,const vector<T>&);
template<typename T,typename Seq,typename Comp>ostream &operator<<(ostream&os,priority_queue<T,Seq,Comp>);
template<typename T>ostream &operator<<(ostream&os,queue<T>);
template<typename T>ostream &operator<<(ostream&os,deque<T>);
template<typename T>ostream &operator<<(ostream&os,stack<T>);
template<typename T,size_t N>ostream &operator<<(ostream&os,const array<T,N>&);
template<typename Key,typename Val,typename Comp>ostream &operator<<(ostream&os,const map<Key,Val,Comp>&);
template<typename Key,typename Val,typename Hash>ostream &operator<<(ostream&os,const unordered_map<Key,Val,Hash>&);
template<typename T,typename Comp>ostream &operator<<(ostream&os,const set<T,Comp>&);
template<typename T,typename Comp>ostream &operator<<(ostream&os,const multiset<T,Comp>&);
template<typename T,typename Hash>ostream &operator<<(ostream&os,const unordered_set<T,Hash>&);
template<typename T1,typename T2>
ostream &operator<<(ostream&os,const pair<T1,T2>&a){
  os<<a.first<<' '<<a.second;
  return os;
}
template<size_t pos,typename...Args>
void write_tuple(ostream&os,const tuple<Args...>&a){
  if constexpr(pos<tuple_size<tuple<Args...>>::value){
    if constexpr(pos>0)os<<' ';
    os<<get<pos>(a);
    write_tuple<pos+1>(os,a);
  }
}
template<typename...Args>
ostream &operator<<(ostream&os,const tuple<Args...>&a){
  write_tuple<0>(os,a);
  return os;
}
template<typename T>
ostream &operator<<(ostream&os,const vector<T>&a){
  os<<'{';
  for(int i=0;i<(int)a.size();i++){
    os<<a[i];
    if(i+1!=a.size())os<<',';
  }
  os<<'}';
  return os;
}
template<typename T,typename Seq,typename Comp>
ostream &operator<<(ostream&os,priority_queue<T,Seq,Comp>a){
  os<<'{';
  if(!a.empty()){
    os<<a.top();a.pop();
    while(!a.empty()){
      os<<',';
      os<<a.top();
      a.pop();
    }
  }
  os<<'}';
  return os;
}
template<typename T>
ostream &operator<<(ostream&os,queue<T>a){
  os<<'{';
  if(!a.empty()){
    os<<a.front();a.pop();
    while(!a.empty()){
      os<<',';
      os<<a.front();
      a.pop();
    }
  }
  os<<'}';
  return os;
}
template<typename T>
ostream &operator<<(ostream&os,deque<T>a){
  os<<'{';
  if(!a.empty()){
    os<<a.front();a.pop_front();
    while(!a.empty()){
      os<<',';
      os<<a.front();
      a.pop_front();
    }
  }
  os<<'}';
  return os;
}
template<typename T>
ostream &operator<<(ostream&os,stack<T>a){
  os<<'{';
  if(!a.empty()){
    os<<a.top();a.pop();
    while(!a.empty()){
      os<<',';
      os<<a.top();
      a.pop();
    }
  }
  os<<'}';
  return os;
}
template<typename T,size_t N>
ostream &operator<<(ostream&os,const array<T,N>&a){
  os<<'{';
  for(int i=0;i<(int)a.size();i++){
    os<<a[i];
    if(i+1!=a.size())os<<',';
  }
  os<<'}';
  return os;
}
template<typename Key,typename Val,typename Comp>
ostream &operator<<(ostream&os,const map<Key,Val,Comp>&a){
  if(a.empty()){
    os<<"{}";
    return os;
  }
  auto itr=a.begin();
  os<<"{["<<itr->first<<","<<itr->second<<']';
  while(++itr!=a.end())os<<",["<<itr->first<<','<<itr->second<<']';
  os<<'}';
  return os;
}
template<typename Key,typename Val,typename Hash>
ostream &operator<<(ostream&os,const unordered_map<Key,Val,Hash>&a){
  if(a.empty()){
    os<<"{}";
    return os;
  }
  auto itr=a.begin();
  os<<"{["<<itr->first<<","<<itr->second<<']';
  while(++itr!=a.end())os<<",["<<itr->first<<','<<itr->second<<']';
  os<<'}';
  return os;
}
template<typename T,typename Comp>
ostream &operator<<(ostream&os,const set<T,Comp>&a){
  if(a.empty()){
    os<<"{}";
    return os;
  }
  auto itr=a.begin();
  os<<'{'<<*itr;
  while(++itr!=a.end())os<<','<<*itr;
  os<<'}';
  return os;
}
template<typename T,typename Comp>
ostream &operator<<(ostream&os,const multiset<T,Comp>&a){
  if(a.empty()){
    os<<"{}";
    return os;
  }
  auto itr=a.begin();
  os<<'{'<<*itr;
  while(++itr!=a.end())os<<','<<*itr;
  os<<'}';
  return os;
}
template<typename T,typename Hash>
ostream &operator<<(ostream&os,const unordered_set<T,Hash>&a){
  if(a.empty()){
    os<<"{}";
    return os;
  }
  auto itr=a.begin();
  os<<'{'<<*itr;
  while(++itr!=a.end())os<<','<<*itr;
  os<<'}';
  return os;
}
#endif
using namespace std;
using ll=long long;
using ull=unsigned long long;
using P=pair<ll,ll>;
template<typename T>using minque=priority_queue<T,vector<T>,greater<T>>;
template<typename T>bool chmax(T &a,const T &b){return (a<b?(a=b,true):false);}
template<typename T>bool chmin(T &a,const T &b){return (a>b?(a=b,true):false);}
template<typename T1,typename T2>void operator++(pair<T1,T2>&a,int){a.first++,a.second++;}
template<typename T1,typename T2>void operator--(pair<T1,T2>&a,int){a.first--,a.second--;}
template<typename T>void operator++(vector<T>&a,int){for(auto &i:a)i++;}
template<typename T>void operator--(vector<T>&a,int){for(auto &i:a)i--;}
using vref=typename vector<bool>::reference;
vref operator|=(vref a,bool b){a=a|b;return a;}
vref operator&=(vref a,bool b){a=a&b;return a;}
vref operator^=(vref a,bool b){a=a^b;return a;}
#define overload3(_1,_2,_3,name,...) name
#define rep1(i,n) for(int i=0;i<(int)(n);i++)
#define rep2(i,l,r) for(int i=(int)(l);i<(int)(r);i++)
#define rep(...) overload3(__VA_ARGS__,rep2,rep1)(__VA_ARGS__)
#define reps(i,l,r) rep2(i,l,r)
#define all(x) x.begin(),x.end()
#define pcnt(x) __builtin_popcountll(x)
#define fin(x) return cout<<(x)<<'\n',static_cast<void>(0)
#define yn(x) cout<<((x)?"Yes\n":"No\n")
#define uniq(x) sort(all(x)),x.erase(unique(all(x)),x.end())
template<typename T>
inline int fkey(vector<T>&z,T key){return lower_bound(z.begin(),z.end(),key)-z.begin();}
ll myceil(ll a,ll b){return (a+b-1)/b;}
template<typename T,size_t n,size_t id=0>
auto vec(const int (&d)[n],const T &init=T()){
  if constexpr (id<n)return vector(d[id],vec<T,n,id+1>(d,init));
  else return init;
}
#ifdef LOCAL
#include<debug.h>
#define SWITCH(a,b) (a)
#else
#define debug(...) static_cast<void>(0)
#define debugg(...) static_cast<void>(0)
#define SWITCH(a,b) (b)
#endif
struct Timer{
  clock_t start;
  Timer(){
    start=clock();
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout<<fixed<<setprecision(16);
  }
  inline double now(){return (double)(clock()-start)/1000;}
  #ifdef LOCAL
  ~Timer(){
    cerr<<"time:";
    cerr<<now();
    cerr<<"ms\n";
  }
  #endif
}timer;
void SOLVE();
int main(){
  int testcase=1;
  //cin>>testcase;
  for(int i=0;i<testcase;i++){
    SOLVE();
  }
}
#include<type_traits>
//min/max f(x) | l<=x<r
template<typename Key,bool MIN=true,typename Func>
std::enable_if_t<std::is_integral_v<Key>,std::pair<Key,decltype(std::declval<Func>()(std::declval<Key>()))>>golden_search(Key l,Key r,const Func&f){
  using Value=decltype(std::declval<Func>()(std::declval<Key>()));
  assert(l<r);
  r--;
  Key a=l-1,b,x;
  Key s=1,t=1;
  while(t<=r-l+1)std::swap(s+=t,t);
  x=a+t-s,b=a+t;
  Value fx=f(x),fy;
  while(x-a!=b-x){
    Key y=a+b-x;
    if(r<y||(fy=f(y),MIN?fx<fy:fx>fy))b=a,a=y;
    else a=x,x=y,fx=fy;
  }
  return std::make_pair(x,fx);
}
//min/max f(x) | l<=x<=r
template<typename Key,bool MIN=true,int loop=100,typename Func>
std::enable_if_t<std::is_floating_point_v<Key>,std::pair<Key,decltype(std::declval<Func>()(std::declval<Key>()))>>golden_search(Key l,Key r,const Func&f){
  using Value=decltype(std::declval<Func>()(std::declval<Key>()));
  assert(l<=r);
  constexpr Key s=0.3819660112501051;
  Key dx=(r-l)*s;
  Value fx=f(l+dx),fy;
  for(int i=0;i<loop;i++){
    fy=f(r-dx);
    if(MIN==(fx<fy))r-=dx,std::swap(l,r);
    else l+=dx,fx=fy;
    dx=(r-l)*s;
  }
  return std::make_pair(l,fx);
}
constexpr ll inf=4e18;
constexpr int step=200;
void SOLVE(){
  int n;
  ll d;
  cin>>n>>d;
  vector<pair<ll,ll>>a(n);
  cin>>a;
  sort(all(a),[](auto lhs,auto rhs){return lhs.second<rhs.second;});
  ll ans=inf;
  auto f=[&](ll c)->ll {
    ll res=c;
    rep(i,n){
      ll p=a[i].second-c;
      if(p>0)res+=a[i].first*((p+d-1)/d);
      if(res>inf)return inf-c;
    }
    chmin(ans,res);
    return res;
  };
  ll l=-1,r=(ll)1e9+1;
  while(r-l>step){
    debug(l,r);
    ll width=(r-l)/step;
    vector<int>pos;
    vector<ll>val;
    rep(i,1,step){
      ll c=(l*(step-i)+r*i)/step;
      pos.push_back(c);
      val.push_back(f(c));
    }
    ll c=pos[min_element(all(val))-val.begin()];
    l=max(-1ll,c-width*40),r=min<ll>((ll)1e9+1,c+width*40);
  }
  rep(i,200)chmin(ans,f(i));
  rep(i,200)if(a.back().second-i>=0)chmin(ans,f(a.back().second-i));
  rep(i,l+1,r)chmin(ans,f(i));
  cout<<ans<<endl;
}
//   ll ans=a[n-1].second;
//   {
//     ll sum=0;
//     rep(i,n){
//       sum+=a[i].first*((a[i].second+d-1)/d);
//       if(sum>ans)break;
//     }
//     chmin(ans,sum);
//   }
//   auto f=[&](int i){
//     ll b=a[i].second;
//     ll res=b;
//     rep(j,i+1,n)res+=a[j].first*((a[j].second-b+d-1)/d);
//     return res;
//   };
//   int id=golden_search<ll>(0,n,f).first;
//   rep(d,-step0,step1){
//     int i=id+d;
//     if(0<=i&&i<n)chmin(ans,f(i));
//   }
//   cout<<ans<<endl;
// }