#include <iostream>
#include <fstream>
#define mod 123457
using namespace std;
ifstream f("branza.in");
ofstream g("branza.out");

int nsapt,taxa,expira;
int cost[100001];
int cerere[100001];
long long ctotal;
int eco(int sapt)
{
    int mn=cost[sapt];
    int fisc=taxa;
    for(int i=sapt-1;i and i>=sapt-expira;i--)
    if(fisc<cerere[i])
    {
        int cat=fisc+cost[i];
        mn=min(mn,cat);
        fisc+=taxa;
    }
    else return mn;
    return mn;
}
int main()
{
    f>>nsapt>>taxa>>expira;
    for(int i=1;i<=nsapt;++i)
        f>>cost[i]>>cerere[i];
    for(int i=1;i<=nsapt;++i)
            ctotal+=eco(i)*cerere[i];
        g<<ctotal;
    return 0;
}