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#include<bits/stdc++.h>
using namespace std;

int n,m;
long long ans;
pair<long long,long long> a[100005];

int main(){

    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);

    cin>>n>>m;
    for(int i=1;i<=n;i++){
        cin>>a[i].first>>a[i].second;
    }
    sort(a+1,a+1+n);
    for(int i=1;i<=n;i++){
        if(m==0)break;
        if(m>a[i].second){
            ans+=a[i].first*a[i].second;
            m-=a[i].second;
        }else{
            ans+=a[i].first*m;
            m=0;
        }
    }
    cout<<ans<<endl;

    return 0;
}

Measured memory_bytes: 1792