#include <fstream>
using namespace std;
ifstream in("branza.in");
ofstream out("branza.out");
int t,v[100001],d[100001],v2[100001],st,dr=-1,k;
inline void stanga(int i)
{
    if(i-k==d[st])
        st++;
}
inline void dreapta(int i)
{
    while(st<=dr&&v[i]*v2[i]<=v[d[dr]]*v2[i]+v2[i]*t*(i-d[dr]))
        dr--;
    d[++dr]=i;
}
int main()
{
    int n,i,s=0;
    in>>n>>t>>k;
    for(i=1; i<=n; i++)
    {
        in>>v[i]>>v2[i];
        dreapta(i);
        if(i>=k)
        {
        stanga(i);
        }
            s+=v[d[st]]*v2[i]+v2[i]*t*(i-d[st]);
    }
    out<<s;
    return 0;
}