#include <iostream>
#include <cstdio>
#include <deque>
#define Nmax 100010
using namespace std;

int c[Nmax],p[Nmax];
deque < int > Q;
int n,k,s1=0,s;
void citire()
{

    scanf("%d %d %d",&n,&s,&k);
    for(int i=1;i<=n;i++)
    {
        scanf("%d",&c[i]);
        scanf("%d",&p[i]);
    }
     for(int i=1;i<k;i++)
    {
        while(!Q.empty()&&c[i]<=c[Q.back()]+s*(i-Q.front()))
            {
                Q.pop_back();
            }
        Q.push_back(i);
        s1+=(c[Q.front()]+s*(i-Q.front()))*p[i];
    }
    for(int i=k;i<=n;i++)
    {
        if(Q.front()==(i-k)&&!Q.empty())
            Q.pop_front();
        while(!Q.empty()&&c[i]<=c[Q.back()]+s*(i-Q.front()))
            {
                Q.pop_back();
            }
        Q.push_back(i);
        s1+=(c[Q.front()]+s*(i-Q.front()))*p[i];
    }
    printf("%d",s1);
}
int main()
{
    freopen("branza.in","r",stdin);
    freopen("branza.out","w",stdout);
    citire();
    //cout << "Hello world!" << endl;
    return 0;
}