#include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <string>
#include <cstring>
#include <cstdlib>
#include <memory.h>
#include <iomanip>
#include <ctime>
#include <cmath>

using namespace std;

const int inf = 1e9;
const int maxh = 1e5;

pair< long long, long long > ct[maxh + 5];

int main()
{
    long long n, m;
    int h;
    cin >> n >> m >> h;
    //scanf("%I64d%I64d%d", &n, &m, &h);
    //scanf("%lld%lld%d", &n, &m, &h);
    for (int i = 0; i < h; ++i)
        //scanf("%I64d%I64d", &ct[i].second, &ct[i].first);
        //scanf("%lld%lld", &ct[i].second, &ct[i].first);
        cin >> ct[i].second >> ct[i].first;

    sort(ct, ct + h);
    long long cnt = 0;
    long long ans = 0;
    for (int i = 0; i < h; ++i)
    {
        if (cnt + ct[i].second < n * m)
            ans += ct[i].second * ct[i].first, cnt += ct[i].second;
        else
        {
            ans += (n * m - cnt) * ct[i].first;
            cout << ans << "\n";
            return 0;
        }
    }
    cout << "Impossible\n";
    return 0;
}