#include <bits/stdc++.h>
using namespace std;

priority_queue<int, vector<int>, greater<int>> prioritati;
priority_queue<pair<int, int>> intervale;

ifstream f("timbre.in");
ofstream g("timbre.out");

int main()
{

  int n,m,k;
  f>>n>>m>>k;

  int mi,c;
  for(int i=1;i<=m;i++){
      f>>mi;
      f>>c;
      intervale.push({mi, c});
  }

  int cost=0;
  while(n>0){

      while(!intervale.empty() && intervale.top().first>=n){

          prioritati.push(intervale.top().second);
          intervale.pop();
      }

      cost+=prioritati.top();
      prioritati.pop();

      n-=k;
  }

  g<<cost;


  return 0;
}