import java.util.Arrays;
import java.util.Comparator;
import java.util.Scanner;

public class Main {
    static final Scanner sc  = new Scanner(System.in);
    static final int MOD = (int) 1E9 + 7;
    static final int INF = Integer.MAX_VALUE;
    
    public static void main(String[] args) {
        int N = nint();
        int M = nint();
        
        Pair[] ps = new Pair[N];
        for (int i = 0; i < N; i++) {
            ps[i] = new Pair(nint(), nint());
        }
        
        Arrays.sort(ps, Pair.A);
        long ans = 0;
        int m = M;
        for (int i = 0; i < N; i++) {
            long a = ps[i].a;
            int b = Math.min(ps[i].b, m);
            ans += a*b;
            m -= b;
            if (m <= 0) break;
        }
        System.out.println(ans);
    }
    
    static class Pair {
        long a;
        int b;
        public long getA() {return a;}
        
        public Pair(long a, int b) {
            super();
            this.a = a;
            this.b = b;
        }
        
        static Comparator<Pair> A = Comparator.comparing(Pair::getA);
        
        @Override
        public String toString() {
            return "a = " + a + ", b = " + b;
        }
    }
    
    private static int nint() {
        return sc.nextInt();
    }

    private static int[] nints(int n) {
        return nints(n, 0, 0);
    }

    private static int[] nints(int n, int padL, int padR) {
        int[] a = new int[padL + n + padR];
        for (int i = 0; i < n; i++)
            a[padL + i] = nint();
        return a;
    }

    private static long nlong() {
        return sc.nextLong();
    }

    private static long[] nlongs(int n) {
        return nlongs(n, 0, 0);
    }

    private static long[] nlongs(int n, int padL, int padR) {
        long[] a = new long[padL + n + padR];
        for (int i = 0; i < n; i++)
            a[padL + i] = nlong();
        return a;
    }

    private static double ndouble() {
        return sc.nextDouble();
    }
    
    private static double[] ndoubles(int n) {
        return ndoubles(n, 0, 0);
    }
    
    private static double[] ndoubles(int n, int padL, int padR) {
        double[] d = new double[n + padL + padR];
        for (int i = 0; i < n; i++) {
            d[padL + i] = ndouble();
        }
        return d;
    }

    private static String nstr() {
        return sc.next();
    }

    private static char[] nchars() {
        return sc.next().toCharArray();
    }

    private static char[] nchars(int padL, int padR) {
        char[] temp = sc.next().toCharArray();
        char[] ret = new char[temp.length + padL + padR];
        System.arraycopy(temp, 0, ret, padL, temp.length);
        return ret;
    }
    
    private static char[][] nchars2(int h, int w) {
        return nchars2(h, w, 0, 0);
    }

    private static char[][] nchars2(int h, int w, int padLU, int padRD) {
        char[][] a2 = new char[h + padLU + padRD][w + padLU + padRD];
        for (int i = 0; i < h; i++)
            System.arraycopy(nchars(), 0, a2[padLU + i], padLU, w);
        return a2;
    }
    
    private static long min(long... ls) {
        return Arrays.stream(ls).min().getAsLong();
    }
    
    private static long max(long... ls) {
        return Arrays.stream(ls).max().getAsLong();
    }
    
    private static long abs(long a, long b) {
        return Math.abs(a - b);
    }
}


Measured memory_bytes: 95112