#include<fstream>
using namespace std;
int n,s,t,a[100001],b[100001],nr,cost,m,i,j,costdz,nror,sum;
int main()
{
    ifstream f("branza.in");
    ofstream g("branza.out");
    f>>n>>s>>t;
    for(i=1;i<=n;i++)
    f>>a[i]>>b[i];
    m=n-t;
    for(i=1;i<=m;i++)
    cost=cost+a[i]*b[i];
    for(i=m+1;i<=n;i++)
    nr=nr+b[i];
    cost=cost+a[m+1]*nr;
    nror=m-1;
    while(nror)
    {
        for(i=m+2;i<=n;i++)
           {
               sum=sum+b[i];
           }
           cost=cost+sum*s;
           m++;
              {
                   sum=0;
                   for(i=m+2;i<=n;i++)
                   sum=sum+b[i];
                   cost=cost+sum*s;
                   m++;
              }
           nror--;
    }

    g<<cost<<" ";
    return 0;
}