#include <iostream>
#include <fstream>
using namespace std;
int cantitate[10000001],pret[10000001],suma;
ifstream f ("branza.in");
ofstream g ("branza.out");
int main()
{
    int N,S,T;
    f>>N>>S>>T;
    for(int i = 0; i <N; ++i)
        f>>pret[i]>>cantitate[i];

    for(int i = 0; i <N; ++i)
       {

        int mini=pret[i]*cantitate[i];
    int S1=S;
    for(int j = i-1 ; j>=i-T && j>=0; j--)
    {

        if(mini>pret[j]*cantitate[i]+cantitate[i]*S1)
        {
           mini= pret[j]*cantitate[i]+cantitate[i]*S1;
        }
        S1*=2;

        }
    suma+=mini;
       }
       g<<suma;
}