#include <bits/stdc++.h>
 
using namespace std;
 
// ifstream fin("intrare.txt");
// ofstream fout("iesire.txt");
 
ifstream fin("branza.in");
ofstream fout("branza.out");
 
int main() {
    int n, s, t, cost_minim = 0;
    fin >> n >> s >> t;
 
    deque<pair<int, int> >deq;
 
    int c, q;   // cost, cantitate
    for (int i = 0; i < n; i++) {
        fin >> c >> q;
        deq.push_back({c, q});
        if (i > t)
            deq.pop_front();
 
        int cost = c;
        deque<pair<int, int>>::reverse_iterator it = deq.rbegin();
        int j = 0;
 
        while (it != deq.rend()) {
            if ((*it).first + j * s < cost)
                cost = (*it).first + j * s;
            it ++;
            j ++;
        }
 
        cost_minim += cost * q;
        // fout << "Saptamana " << i+1 << ": " << cost << '\n';
    }
 
    fout << cost_minim;
    return 0;
}