#include <iostream>
#include <fstream>

using namespace std;

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

int main()
{
    int n, s, t;
    f >> n >> s >> t;

    int m[n][2];
    for (int i =0; i< n; i++){
        f >> m[i][0] >> m[i][1];
    }

    int total=0;
    for (int i = 0; i < n;i ++){
        if (m[i][1]){
            total += m[i][0] * m[i][1];
            m[i][1] = 0;
        }
        for (int j = 0; j < t; j++){
            if (i+j+1 < n){
                if (m[i][0]+(j+1)*s < m[i+j+1][0]){
                    m[i+j+1][0] = m[i][0]+(j+1)*s;
                }
            }
        }
    }

    g << total;

    return 0;
}