#include <fstream>
#include <deque>
#include <utility>
#include <vector>

using namespace std;

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

deque < pair <int,int> > dq;
int n,p,t,i,c,x,cost;

int main()
{
    fin>>n>>p>>t;
    for(i=1;i<=n&&i<=t;i++)
    {
        fin>>c>>x;
        if(!dq.empty()){
        int q=dq.back().first;
        int w=dq.back().second;}
        while(!dq.empty()&&dq.back().first+p*(i-dq.back().second)>c)
        {
            dq.pop_back();
        }
        dq.push_back(make_pair(c,i));
        int q=dq.front().first;
        int w=dq.front().second;
        cost+=(q+p*(i-w))*x;
    }
    for(i=t+1;i<=n;i++)
    {
        fin>>c>>x;
        if(!dq.empty()){
        int q=dq.back().first;
        int w=dq.back().second;}
        while(!dq.empty()&&dq.back().first+p*(i-dq.back().second)>c)
        {
            dq.pop_back();
        }
        dq.push_back(make_pair(c,i));
        int q=dq.front().first;
        int w=dq.front().second;
        cost+=(q+p*(i-w))*x;
    }
    fout<<cost<<'\n';
    return 0;
}