#include <fstream>
using namespace std;
ifstream in("branza.in");
ofstream out("branza.out");
long long 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]<=v[d[dr]]+t*(i-d[dr]))
        dr--;
    d[++dr]=i;
}

void scrie()
{
    for (int i = st; i <= dr; i++)
    {
        out << d[i] << " ";
    }
    out << "\n";
}

int main()
{
    int n,i;
    long long s=0;
    in>>n>>t>>k;
    for(i=1; i<=n; i++)
    {
        in>>v[i]>>v2[i];
        dreapta(i);
        if(i>=k)
        {
            stanga(i);
        }
        //scrie();
        s+=v[d[st]]*v2[i]+v2[i]*t*(i-d[st]);
    }
    out<<s;
    return 0;
}