#include "bits/stdc++.h"
using namespace std;
#define int long long
//#define endl '\n'
int mod=1e9+7;
int mod2=998244353;
const int INF=1e9;

signed main(){
  int n,m;
  cin>>n>>m;
  vector<pair<int,int> > ab(n);
  for(int i=0;i<n;i++){
    int a,b;
    cin>>a>>b;
    ab[i]=make_pair(a,b);
  }
  sort(ab.begin(),ab.end());
  int ans=0;
  int cnt=0;
  int k=0;
  while(cnt<m){
    int h=min(m-cnt,ab[k].second);
    ans+=ab[k].first*h;
    cnt+=h;
    k++;
  }
  cout<<ans<<endl;
}

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