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

ifstream f("branza.in");
ofstream g("branza.out");

int main() {
    int n = 5, taxa = 10, termen = 3, pret, kg, v[100001], best = 0, cost = 0;
    f>>n>>taxa>>termen;
    f>>pret>>kg;
    v[0] = pret;
    cost = pret * kg;
    for(int i = 1; i < n; ++i){
        f>>v[i]>>kg;
        if((v[best] + taxa * (i-best)) > v[i] || (i-best == termen) ){
            v[best] = v[i];
            best = i;
        }
        cost += kg * v[best] + taxa * (i-best) * kg;
    }
    g<<cost;
    return 0;
}