#include <iostream>
#include <fstream>

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

int N, S, T, c[100001], p[100001], cost, i, j, t1, v[100001], x;

int main()
{
    f>>N>>S>>T;
    for(i=1; i<=N; i++)
        f>>c[i]>>p[i];

    for(i=1; i<=N; i++)
    {
        if (i + T <= N)
            t1 = i + T;
        else t1 = N;

        for (j=i; j<=t1; j++)
        {
            x = (c[i] + S*(j-i))*p[j];
            if (x < v[j] || v[j] == 0)
                v[j] = x;

        }
    }

    for(i=1; i<=N; i++)
        cost += v[i];
    g<<cost;
}