#define _CRT_SECURE_NO_WARNINGS
#define MATH_PI 3.14159265358979323846264338327950288419716939
#include <iostream>
#include <string.h>
using namespace std;
/* 入力省略用汎用マクロ・クラス */
#define CLIP(ptr, min, max) {if((min)<=(max)){if(ptr<(min)){ptr=(min);}if(ptr>(max)){ptr=(max);}}}
#define max(a, b) a > b ? a : b;
#define min(a, b) a > b ? b : a;
#define Sin(deg) sin((deg)*MATH_PI/180)
#define Cos(deg) cos((deg)*MATH_PI/180)
#define Tan(deg) tan((deg)*MATH_PI/180)
#define Rad(deg) ((deg)*MATH_PI/180)
#define _for(num) for(int i=0 ; i<num ; ++i)
#define _forj(num) for(int j=0 ; j<num ; ++j)
#define _fork(num) for(int k=0 ; k<num ; ++k)
#define _forl(num) for(int l=0 ; l<num ; ++l)
#define _forn(num) for(int n=0 ; n<num ; ++n)
#define __for(param, num) for(int param=0 ; param<num ; ++param)
/* paiza用の入力データ処理クラス */
class myStr { FILE *fp; int ptr, EOL_flag; int LocalFileOpen(); int LocalFileClose(); int LocalRead(); int GetNextData(int *s, int *e); public:myStr(); ~myStr(); string buf; int bufsize; int StreamRead(); int GetNextAsInt(); long long int GetNextAsllInt(); char *GetNextAsChar(); int isEOL(); void Print(string *str); void Print(string &str); };
#ifdef DEBUGMODE
#include "LocalFunc.h"
#else
myStr::myStr() {}myStr::~myStr() {}
int myStr::StreamRead() { getline(cin, buf); ptr = 0; bufsize = buf.size(); EOL_flag = 0; return 1; }
void wait() {}
#endif
int myStr::GetNextData(int *s, int *e) { int i; for (i = ptr; i < bufsize; ++i) { if (buf[i] != ' ') { *s = i; break; } }if (i >= bufsize) { EOL_flag = 1; return 0; }for (; i < bufsize; ++i) { if (buf[i] == ' ') { break; } }*e = i - 1; ptr = i; return 1; }int myStr::GetNextAsInt() { int s, e; int ret = GetNextData(&s, &e); if (isEOL()) return 0; return atoi(&buf[s]); }long long int myStr::GetNextAsllInt() { int s, e; long long int ret = GetNextData(&s, &e); if (isEOL()) return 0; return atoll(&buf[s]); }char *myStr::GetNextAsChar() { int s, e; int ret = GetNextData(&s, &e); if (isEOL()) return NULL; char *chr; chr = new char[e - s + 2]; int i; for (i = s; i <= e; ++i) { chr[i - s] = buf[i]; }chr[i - s] = 0; return chr; }int myStr::isEOL() { return EOL_flag; }void myStr::Print(string *str) { cout << str->c_str() << endl; }void myStr::Print(string &str) { Print(&str); }
/* 便利系関数 */
#define SORT_ORDER_ASCENDING 1
#define SORT_ORDER_DESCENDING 2
int GetMax(int *arr, int arr_size, int if_equal_get_first_flag = 1, int *ret_index = NULL) { if (arr_size <= 0) abort(); int mx = arr[0], index = 0; for (int n = 1; n < arr_size; ++n) { if (if_equal_get_first_flag && arr[n] > mx || !if_equal_get_first_flag && arr[n] >= mx) { index = n; mx = arr[n]; } }if (*ret_index) *ret_index = index; return mx; }int GetMin(int *arr, int arr_size, int if_equal_get_first_flag = 1, int *ret_index = NULL) { if (arr_size <= 0) abort(); int mn = arr[0], index = 0; for (int n = 1; n < arr_size; ++n) { if (if_equal_get_first_flag && arr[n] < mn || !if_equal_get_first_flag && arr[n] <= mn) { index = n; mn = arr[n]; } }if (*ret_index) *ret_index = index;	return mn; }template<typename T> void bSort(T *arr, int arr_size, int order, int *rule1 = NULL, int *rule2 = NULL, int *rule3 = NULL, int *rule4 = NULL) { int swapped, flag; T tmp; while (true) { swapped = 0; for (int n = 0; n < arr_size - 1; ++n) { flag = 0; if (order == SORT_ORDER_ASCENDING) { if (!rule1) { if (arr[n] > arr[n + 1]) flag = 1; } else { if (rule1[n] > rule1[n + 1]) flag = 1; if (rule1[n] == rule1[n + 1] && rule2) { if (rule2[n] > rule2[n + 1]) flag = 1; if (rule2[n] == rule2[n + 1] && rule3) { if (rule3[n] > rule3[n + 1]) flag = 1; if (rule3[n] == rule3[n + 1] && rule4) { if (rule4[n] > rule4[n + 1]) flag = 1; } } } } }if (order == SORT_ORDER_DESCENDING) { if (!rule1) { if (arr[n] < arr[n + 1]) flag = 1; } else { if (rule1[n] < rule1[n + 1]) flag = 1; if (rule1[n] == rule1[n + 1] && rule2) { if (rule2[n] < rule2[n + 1]) flag = 1; if (rule2[n] == rule2[n + 1] && rule3) { if (rule3[n] < rule3[n + 1]) flag = 1; if (rule3[n] == rule3[n + 1] && rule4) { if (rule4[n] < rule4[n + 1]) flag = 1; } } } } }if (flag) { tmp = arr[n]; arr[n] = arr[n + 1]; arr[n + 1] = tmp; if (rule1) swap(rule1[n], rule1[n + 1]); if (rule2) swap(rule2[n], rule2[n + 1]); if (rule3) swap(rule3[n], rule3[n + 1]); if (rule4) swap(rule4[n], rule4[n + 1]); swapped = 1; } }if (!swapped) break; } }int Within(int px, int py, int x1, int y1, int x2, int y2, int allow_on_border = 1) { if (allow_on_border) { if (px >= x1 && px <= x2 && py >= y1 && py <= y2) return 1; else return 0; } else { if (px > x1 && px < x2 && py > y1 && py < y2) return 1; else return 0; } }
inline int separate(char *c1, char *c2, char *c3, char *c4) { unsigned int i; char *src, *s1, *s2; int flag; src = (char *)malloc(strlen(c1) + 1); s1 = (char *)malloc(strlen(c2) + 1); s2 = (char *)malloc(strlen(c2) + 1); strcpy(src, c1); strcpy(s2, c2); c3[0] = 0; c4[0] = 0; flag = 0; for (i = 0; i < strlen(src); i++) { strncpy(s1, src + i, strlen(s2)); s1[strlen(s2)] = 0; if (strcmp(s1, s2) == 0) { strcpy(c3, src); c3[i] = 0; strcpy(c4, src + i + strlen(s2)); flag = 1; break; } }free(s1); free(s2); if (flag) { free(src); return 1; } else { strcpy(c3, src); free(src); c4[0] = 0; return 0; } }
/****************************************************************/
/* 問題固有のクラス
/****************************************************************/
/****************************************************************/
/* メイン処理部
/****************************************************************/
int main(void) {

	char *cbuf;
	myStr str;

	int N, M;
	int Shop[100001][2];

	str.StreamRead();
	N = str.GetNextAsInt();
	M = str.GetNextAsInt();

	_for(N){
		str.StreamRead();
		Shop[i][0] = str.GetNextAsInt();
		Shop[i][1] = str.GetNextAsInt();
	}

	int flag;
	int h = N / 1.3;
	int i, j;
	int prm;
	long long int total;
	while(true){
		flag = 0;
		if(N == 1) break;
		for(i = 0 ; i + h < N ; ++i){
			if(Shop[i][0] > Shop[i+h][0]){
				prm = Shop[i][0];
				Shop[i][0] = Shop[i + h][0];
				Shop[i + h][0] = prm;
				prm = Shop[i][1];
				Shop[i][1] = Shop[i + h][1];
				Shop[i + h][1] = prm;
				flag = 1;
			}
		}
		if(h > 1) h = h / 1.3;
		if(!flag && h == 1) break;
	}

	total = 0;
	int left = M;
	_forn(N){
		if(Shop[n][1] <= left){
			total += (long long int)Shop[n][0] * Shop[n][1];
			left -= Shop[n][1];
		}
		else if (Shop[n][1] > left) {
			total += (long long int)Shop[n][0] * left;
			left = 0;
		}
		if(left == 0) break;
	}

	cout << total << endl;

	wait();

	return 0;
}



Measured memory_bytes: 1024