#include <iostream>

#include <fstream>

using namespace std;



ifstream f("branza.in");

ofstream g("branza.out");



int main() {

    long long n = 5, taxa = 10, termen = 3, pret, kg, best = 0, cost = 0, best_value;

    f>>n>>taxa>>termen;


    termen++;

    f>>pret>>kg;



    cost = pret * kg;

    best_value = pret;

    for(int i = 1; i < n; ++i){

        f>>pret>>kg;



        if(i-best >= termen){

            best_value = pret;

            best = i;

        }

        if(best_value + taxa * (i-best) > pret && best != i){

            best_value = pret;

            best = i;

        }



       // cout<<pret<<' '<<best_value<<' '<<i<<endl;

        cost += kg * best_value + taxa * (i-best) * kg;
//        cout<<best_value<<' '<<termen<<endl;

    }

    g<<cost;



    return 0;

}