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

using namespace std;

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