#include <bits/stdc++.h>
using namespace std;
int main() {
    long long V, tempV, n;
    long long maxValue = 0, ans = 0;
    map<int, long long> water;
    cin >> V >> n;

    for (int i = 0; i < n; i++) {
        long long nowValue, nowVolume;
        cin >> nowVolume >> nowValue;
        if (!water[nowValue])
            water[nowValue] = nowVolume;
        else
            water[nowValue] += nowVolume;
        if (nowValue > maxValue)
            maxValue = nowValue;
    }
    for (map<int, long long>::iterator iter = water.end(); tempV < V; iter--) {
        if (iter == water.end())
            continue;
        int addV = min(V - tempV, iter->second);
        tempV += addV;
        ans += addV * iter->first;
        if (iter == water.begin())
            break;
    }
    cout << ans << endl;
}