//
//  main.cpp
//  Branza
//
//  Created by Darius Buhai on 20/10/2018.
//  Copyright © 2018 Darius Buhai. All rights reserved.
//

#include <iostream>
#include <deque>
#include <fstream>

using namespace std;

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

struct sapt{
    int c, p;
};

int n, t, s, x, ct;
deque<sapt> b;

void rez_branza()
{
    fin>>n>>s>>t;
    for(int i=0;i<n;i++)
    {
        sapt xx;
        fin>>xx.c;
        fin>>xx.p;
        b.push_back(xx);
        if(b.size()==1){
            ct+=b.back().c;
            continue;
        }
        if(b.size()>t)
            b.pop_front();
        int cp = xx.c * xx.p, cc, ii=1;
        for(auto it : b){
            cc = it.c*it.p + s*ii;
            if(cc < cp)
                cp = cc;
            ii++;
        }
        ct+=cp;
    }
    cout<<ct;
    fout<<ct;
}

int main() {
    rez_branza();
    return 0;
}