/*
TASK:stairs
LANG:c
*/

#include<stdio.h>

int n, k;
float zlato = 0;
float buffer[101][2] = {0};
float min;

int main()
{
          int i, j, ind;
          float a, b;

          scanf("%d %d", &n, &k);

          for(i = 0; i < n; i++)
          {
                scanf("%f %f", &a, &b);
                buffer[i][0] = a;
                buffer[i][1] = b;
          }
          for(;;)
          {
                  min = 1000000000;
                  for(i = 1; i < n; i++)
                  {
                        if(buffer[i-1][1] * buffer[i][0] < min){min = buffer[i-1][1] * buffer[i][0]; ind = i;}        
                  }
                  
                  zlato = zlato + min;
                  n--;
                  if(n == k) break;
                  buffer[ind-1][0] = buffer[ind-1][0] + buffer[ind][0];
                  buffer[ind-1][1] = buffer[ind-1][1] + buffer[ind][1];
                  
                  for(i = ind; i < n; i++)
                  {
                          buffer[i][0] = buffer[i+1][0];
                          buffer[i][1] = buffer[i+1][1];
                  }
          }
                           
          printf("%.3f\n", zlato);
          
          return 0;
}







