#include <bits/stdc++.h>

using namespace std;
ifstream fin("branza.in");
ofstream fout("branza.out");
deque <int> d;
int n,s,t,p,c[100001],mn;
unsigned long long sum=0;

int main()
{
    fin>>n>>s>>t;
    for(int i=1;i<t;i++)
    {
        fin>>c[i]>>p;
        sum+=c[i]*p;
        while(!d.empty()&&c[d.back()]>=c[i])
            d.pop_back();
        d.push_back(i);
    }
    for(int i=t;i<=n;i++)
    {
        fin>>c[i]>>p;
        while(!d.empty()&&c[d.back()]>=c[i])
            d.pop_back();
        if(d.front()<=i-t)
            d.pop_front();
        mn=c[i]; // pret pe kg
        for(deque<int>::iterator j=d.begin();j<d.end();j++)
        {
            mn=min(mn,c[*j]+s*(i-*j));
        }
        d.push_back(i);
        sum+=mn*p;
    }
    fout<<sum;
}