//
//
//  Codeforces: ganariya
//  AtCoder: ganariya2525
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//  MMMMMMMMMMMMMMMMNOtXtdMMMMMNxzZMMNmgHyVVVVVW%..?/(.,(x-..........(HfffpffpffpffWHHffpffppfVyyWMMMMMM


//include
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <unordered_map>
#include <climits>
#include <set>
#include <unordered_set>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <queue>
#include <random>
#include <complex>
#include <regex>
#include <locale>
#include <random>
#include <type_traits>

using namespace std;

#define SHOW_VECTOR(v) {std::cerr << #v << "\t:";for(const auto& xxx : v){std::cerr << xxx << " ";}std::cerr << "\n";}
#define SHOW_MAP(v){std::cerr << #v << endl; for(const auto& xxx: v){std::cerr << xxx.first << " " << xxx.second << "\n";}}

using LL = long long;

//~~~~~~~~~~~~~~~~~~~~~_(＾～＾ ｣ ∠)_~~~~~~~~~~~~~~~~~~~~~


int main() {

    int N, M;
    cin >> N >> M;

    vector<LL> A(N), B(N);
    for (int i = 0; i < N; i++) cin >> A[i] >> B[i];

    vector<pair<LL, LL>> AB(N);
    for (int i = 0; i < N; i++) AB[i] = make_pair(A[i], B[i]);
    sort(AB.begin(), AB.end());

    LL ans = 0;
    for (int i = 0; i < N; i++) {
        LL r = min(AB[i].second, (LL) M);
        M -= r;
        ans += r * AB[i].first;
    }
    cout << ans;

    return 0;
}
















































Measured memory_bytes: 3328