#include <iostream>
#include <fstream>

using namespace std;

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

int a[100001],b[100001];

int main()
{
    int n,s,t,min,i,j,ct;
    long long sum=0;

    in>>n>>s>>t;
    for(i=0; i<n; i++)
        in>>a[i]>>b[i];
    for(i=0; i<n; i++)
    {
        ct=0;
        min=10000001;
        for(j=i; j>=0&&ct<=t; j--)
        {
            if(a[j]+ct*s<min)
                min=a[j]+ct*s;
            ct++;
        }
        sum=sum+min*b[i];
    }
    out<<sum;

    return 0;
}