#include <fstream>
using namespace std;
ifstream f("branza.in");
ofstream g("branza.out");
int main()
{
    int n, t, s;
    f >> n >> t >> s;
    int pret[n+1];
    int cantitate[n+1];
    int suma = 0, minim, prec;
    for(int i = 0; i < n; i++)
    {
        f >> pret[i] >> cantitate[i];
    }
    for(int i = 0; i < n; i++)
    {
        minim = pret[i] * cantitate[i];
        for(int j = i - 1 ; j >= i - s && j >= 0; j--)
        {
            prec = (pret[j] + t * (i - j)) * cantitate[i];
            if(prec < minim)
                minim = prec;
        }
        suma = suma + minim;
    }
    g << suma;
    return 0;
}