#include <fstream>

using namespace std;

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

int main()
{
    int n, s, t;
    int c, p, m = 0, z = 0, rez = 0;
    in >> n >> s >> t;
    for(int i = 0; i < n; i++)
    {
        in >> c >> p;
        if(!m || z == t || m > c)
        {
            z = 0;
            m = c;
        }
        rez += m * p;
        m += s;
        z++;
    }
    out << rez << '\n';
    return 0;
}