#include<bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int,int>;
const int N = 1e2+10;
int t,n,m,k,l,r,op,x,y;
double sum;
struct nd{
    double k;
    int v;
    bool operator<(const nd&n1)const{
        return k<n1.k;
    }
};
nd f[N];
void solve(){
    while(cin>>n>>t){
        sum = 0;
        for(int i = 1;i<=n;i++){
            cin>>f[i].k>>f[i].v;
        }
        sort(f+1,f+1+n);
        for(int i = 1;i<=n;i++){
            if(t>=f[i].v){
                sum += f[i].v * f[i].k;
                t-= f[i].v;
            }else{
                sum += t * f[i].k;
                t = 0;
                break;
            }
        }
        if(t)sum += t;
        cout<<(int)sum<<"\n";
    }
}
int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);  
    solve();
    return 0;
}