#include <iostream>
#include <fstream>
#include <vector>

using namespace std;

int main() {
    ifstream fin("branza.in");
    ofstream fout("branza.out");
    int N, T;
    unsigned long long int S;
    fin >> N >> S >> T;
    vector<int> cerere(N);
    vector<unsigned long long int> cost(N);
    for(int i=0; i<N; ++i) {
        fin >> cost[i] >> cerere[i];
    }
    for(int i=N-1; i>=0; --i) {
        unsigned long long int c = cost[i];
        for(int j=1; i+j<N && j<T; ++j) {
            c += S;
            //cout << c << '\n';
            if(c < cost[i+j]) {
                cost[i+j] = c;
            } else {
                break;
            }
        }
    }
    unsigned long long int suma = 0;
    for(int i=0; i<N; ++i) {
        suma += 1ULL * cost[i] * cerere[i];
    }
    fout << suma << '\n';
    return 0;
}