#include <iostream>
#include <fstream>
#include <deque>
using namespace std;

ifstream fin("branza.in");
ofstream gout("branza.out");

int main()
{
    deque<int> preturi;
    int z, taxa, termen, pret, kg, cost = 0, cost_total = 0;
    fin>>z>>taxa>>termen;
    for(int i = 0; i < z; ++i){
        fin>>pret>>kg;
        preturi.push_back(pret);
        deque<int> aux = preturi;
        cost = pret * kg;
        for(int j = 0; j < preturi.size(); ++j){
            if(aux.front() * kg + taxa * kg * (preturi.size()-j-1)  < cost){
                cost = aux.front() * kg + taxa * kg * (preturi.size()-j-1);
            }
            aux.pop_front();
        }
        cost_total += cost;

        if(preturi.size() == termen)
            preturi.pop_front();
    }


    gout<<cost_total;
    return 0;
}