/*
TASK: lift
LANG: C
*/

#include<stdio.h>

int tegla[16] = {0};
int viso4ina[16] = {0};

int main()
{

    int ind, p, sbor = 0, masa = 0, i, n, t, min = 32000, j;
    
    scanf("%d %d", &n, &t);
    
    for(i = 0; i < n; i++)
    {
          scanf("%d %d", &viso4ina[i], &tegla[i]);
          if(tegla[i] < min) min = tegla[i];
    }
    
    for(i = 0; i < n; i++)
    {
          masa = masa + tegla[i];
    }
    
    if(t <= min) printf("0\n"); 
    else 
    {
         for(i = 0; i < n; i++)
         {
               for(j = i; j < n; j++)
               {
                     if(min < tegla[j]) {min = tegla[j]; ind = j;} 
               }
               p = tegla[i];
               tegla[i] = min;
               tegla[ind] = p;
         }
         for(i = 0; i < masa / t + 1; i++)
         {
               sbor = sbor + viso4ina[i];
         }
         sbor = sbor + (masa/t)*viso4ina[n-1]; 
         printf("%d\n", sbor);
    }
    
    return 0;
}




