#include <bits/stdc++.h>

using namespace std;

ifstream fin("timbre.in");
ofstream fout("timbre.out");

int n, m, k;
priority_queue<pair<int, int>> pq;
priority_queue<int, vector<int>, greater<>> cost;

int main(){
    int s, c, i;
    long long sol = 0;
    fin >> n >> m >> k;

    for (i = 1; i <= m; i++){
        fin >> s >> c;
        pq.emplace(s, c);
    }

    while (n){
        while (!pq.empty() && pq.top().first >= n){
            cost.push(pq.top().second);
            pq.pop();
        }

        sol += cost.top();
        cost.pop();
        n -= k;

    }

    fout << sol;

    return 0;
}