#define _CRT_SECURE_NO_DEPRECATE
#include <iostream>
#include <string>
#include <vector>
#include <sstream>
#include <queue>
#include <algorithm>
#include <iomanip>
#include <map>
#include <set>
#include <math.h>
#include <stack>
#include <deque>
#include <numeric>
#include <cstring>
#include <cstdio>
#include <cstdlib>
#include <bitset>

#define all(v) v.begin(),v.end()
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
const ld epsylon = 1e-9;
typedef unsigned int ui;
//inline long double get_time(){   
//	return (long double)clock()/CLOCKS_PER_SEC;
//};
//ld start_time,end_time;

int n, p;
int c[102], d[102];

vector <pair<int, int> > left1;

int main()
{
	freopen("summer.in","r",stdin);
	freopen("summer.out","w",stdout);
	//start_time = get_time();
	//program
	scanf("%d %d", &n, &p);
	for (int i = 0; i < n; ++i)
		scanf("%d %d", &d[i], &c[i]);
	long long res = 0;
	int inleft1 = 0;
	for (int i = 0; i < n; ++i)
	{
		for (int j = 0; j < left1.size(); ++j)
		{
			if (c[i] > left1[j].first)
			{
				if (left1[j].second > d[i])
				{
					left1[j].second -= d[i];
					res += d[i] * left1[j].first;
					inleft1 -= d[i];
					d[i] = 0;
				}
				else {
					res += left1[j].second * left1[j].first;
					d[i] -= left1[j].second;
					inleft1 -= left1[j].second;
					left1[j].second = 0;
				}
			}
			else {
				left1[j].first = c[i];
			}
		}
		if (inleft1 < p)
		{
			left1.push_back(make_pair(c[i], p-inleft1));
			inleft1 = p;
		}
		res += d[i] * c[i];
	}
	cout << res << endl;
	//end program
	//end_time=get_time()-start_time;
	return 0;
}