#include<iostream>
#include<fstream>
#include<climits>
using namespace std;

int n,s,C[10010],Y[10010];
int f[10010],Limit[10010];
const int INFI = INT_MAX;


void search(int step,int Save,int Cost);

int main(void){
    int i,j;
    ifstream in("factory.in");
    ofstream out("factory.out");
    
    in>>n>>s;
    for(i=1;i<=n;i++)
        in>>C[i]>>Y[i];
    
    for(i=1;i<=n;i++){
        f[i] = INFI;
        Limit[i] = 0;
        for(j=i;j<=n;j++)
            Limit[i] += Y[j];
    }
    
    search(1,0,0);
    
    out<<f[n];
    
    return 0;
}

void search(int step,int Save,int Cost){
     if(Cost>=f[step]) return;
     
     int t;
     
     if(step!=n){
        if(Save>=Y[step])
           for(t=0;t<=Limit[step]-Save;t++){
               if(f[step]>Cost+Save*s+t*C[step]) f[step] = Cost+Save*s+t*C[step];
               search(step+1,Save-Y[step]+t,Cost+Save*s+t*C[step]);
               }
        else
            for(t=Y[step]-Save;t<=Limit[step]-Save;t++){
                if(f[step]>Cost+Save*s+t*C[step]) f[step] = Cost+Save*s+t*C[step];
                search(step+1,Y[step]-(t+Save),Cost+Save*s+t*C[step]);
            }
     }
     else{
         if(f[n]>Cost+(Y[n]-Save)*C[n]+Save*s)
            f[n]=Cost+(Y[n]-Save)*C[n]+Save*s;
     }
}