#include <bits/stdc++.h>
#include <iostream>
#include <math.h>
using namespace std;
typedef unsigned long long ll;
ll v, n, w;
ll ans = 0;
struct zhu {
    ll a;
    ll b;
};
bool cmp(zhu m, zhu n) { return (m.b) > (n.b); }
int main() {
    cin >> v >> n;
    struct zhu z[n];
    for (int i = 0; i < n; i++) {
        scanf("%llu%llu", &z[i].a, &z[i].b);
    }
    unsigned long long sum = 0;
    // cout<<"hello world";
    sort(z, z + n, cmp);
    int i = 0;
    while (v > 0) {
        // cout<<v<<endl;
        if (z[i].a <= v) {
            sum += z[i].a * z[i].b;
            v -= z[i].a;
        } else {
            sum += v * z[i].b;
        }
        i++;
        if (i >= n) {
            break;
        }
    }

    cout << sum << endl;
}