#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <functional>
#include <stack>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
const int maxn = 2e5 + 5;
struct water(ll a, ll b) {
    ll volume = a;
    ll value = b;
};
bool cmp(water A, water B) {
    return A.value > B.value;  //价值降序
}
water w[maxn];
int main() {
    ll V, n;
    cin >> V >> n;
    ll ans = 0;
    for (int i = 1; i <= n; i++) {
        cin >> w[i].volume >> w[i].price;
    }
    sort(w, w + n, cmp);
    while (V) {
        for (int i = 1; i <= n; i++) {
            if (V > w[i].volume) {
                V -= w[i].volume;
                ans += w[i].volume * w[i].price;
            } else {
                V = 0;
                ans += V * w[i].price;
            }
        }
    }
    cout << ans << endl;
    return 0;
}

/sandbox/1/a.cpp:11:16: error: expected ')' before 'a'
 struct water(ll a, ll b) {
             ~  ^~
                )
/sandbox/1/a.cpp:15:16: error: 'A' has incomplete type
 bool cmp(water A, water B) {
          ~~~~~~^
/sandbox/1/a.cpp:11:8: note: forward declaration of 'struct water'
 struct water(ll a, ll b) {
        ^~~~~
/sandbox/1/a.cpp:15:25: error: 'B' has incomplete type
 bool cmp(water A, water B) {
                   ~~~~~~^
/sandbox/1/a.cpp:11:8: note: forward declaration of 'struct water'
 struct water(ll a, ll b) {
        ^~~~~
/sandbox/1/a.cpp:18:7: error: elements of array 'water w [200005]' have incomplete type
 water w[maxn];
       ^
/sandbox/1/a.cpp:18:7: error: storage size of 'w' isn't known