#include <algorithm>
#include <iostream>
#include <fstream>
using namespace std;
ifstream in("timbre.in");
ofstream out("timbre.out");
const int maxn = 10005;
int H[maxn];
pair <int, int> v[maxn];

/*
inline bool cmp(pair <int, int> a, pair <int, int> b)
{
    return a.second - b.second;
}
*/

int main()
{
    int n, m, k;
    in >> n >> m >> k;
    for(int i = 1; i <= m; i++)
        in >> v[i].first >> v[i].second;
    //sort(v + 1, v + m + 1, cmp);
    long long total = 0;
    while(n)
    {
        k = min(k, n);
        int costmn = (1 << 30);
        int poz = 0;
        for(int i = 1; i <= m; i++)
        {
            if(v[i].first >= k)
            {
                if(costmn > v[i].second)
                {
                    costmn = v[i].second;
                    poz = i;
                }
            }
        }
        n -= k;
        total += costmn;
        v[poz] = make_pair(0, 0);
    }
    out << total;
    return 0;
}