/*
TASK:stairs
LANG:C++
*/

#include <stdio.h>

int main()
{
    int n = 0;
    int k = 0;
    float _sum = 0;
    float matrix[100][3];
    scanf("%d %d\n",&n,&k);
    for (int i = 0; i < n; i++)
    {
      scanf("%f %f",&matrix[i][0],&matrix[i][1]);
    }
    for (int i = 0; i < (n - 1); i++)
    {
      matrix[i][2] = matrix[i][1]*matrix[i+1][0];
    }
    while (k != n)
    {
      float min = 30000;
      int _index = 0;
      for (int i = 0; i < (n - 1); i++)
        if (matrix[i][2] < min) 
        {
          min = matrix[i][2];
          _index = i;
        }
      _sum += min;      
      bool _my = false;
      for (int i = 0; i < (n - 1); i++)
      {
        if (_my) 
        {
          matrix[i][0] = matrix[i+1][0];
          matrix[i][1] = matrix[i+1][1];
          matrix[i][2] = matrix[i+1][2];
        }
        if (i == _index) 
        {
          matrix[i][0] = matrix[i][0] + matrix[i+1][0];
          matrix[i][1] = matrix[i][1] + matrix[i+1][1];
          matrix[i][2] = (matrix[i+2][1] + matrix[i+1][1])*matrix[i+1][1];
          _my = true;       
        }
      }
      n--;   
    }
    printf("%0.3f",_sum);
    scanf("%d %d\n",&n,&k);
}
