#include<bits/stdc++.h>
using namespace std;typedeflonglong ll;typedeflongdouble lf;typedefunsignedlonglong ull;typedef pair<ll,int>P;constint inf =0x7f7f7f7f;const ll INF =1e16;constint N =1e4+10;const ull base =131;const ll mod =1e9+7;constdouble PI =acos(-1.0);constdouble eps =1e-4;inlineintread(){
int x=0,f=1;char ch=getchar();while(ch<'0'||ch>'9'){
if(ch=='-')f=-1;ch=getchar();}while(ch>='0'&&ch<='9'){
x=x*10+ch-'0';ch=getchar();}return x*f;}inline string readstring(){
string str;char s=getchar();while(s==' '||s=='\n'||s=='\r'){
s=getchar();}while(s!=' '&&s!='\n'&&s!='\r'){
str+=s;s=getchar();}return str;}intrandom(int n){
return(int)(rand()*rand())%n;}voidwritestring(string s){
int n = s.size();for(int i =0;i < n;i++){
printf("%c",s[i]);}}
ll fast_power(ll a,ll p){
ll ans =1;while(p){
if(p&1) ans =(ans*a)%mod;
p >>=1;
a =(a*a)%mod;}return ans;}
ll a[N],b[N];intmain(){
//srand((unsigned)time(NULL));//freopen( "out.txt","w",stdout);int n =read(),s =read();
ll ans =0;for(int i =1;i <= n;i++){
a[i]=read();b[i]=read();}int last =1;
ll num = a[1];
ans += a[1]*b[1];for(int i =2;i <= n;i++){
if(b[i]*num+(i-last)*s*b[i]< a[i]*b[i]){
ans += b[i]*num+(i-last)*s*b[i];}else{
ans += a[i]*b[i];
last = i;
num = a[i];}}
cout<<ans<<endl;return0;}