答案（参考）

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main()
{
int n, m;
cin >> n >> m;
vector<pair<int, int>> v;
for(int i = 0; i < m; i++)
{
int a, b;
cin >> a >> b;
v.emplace_back(b, a);
}
// 按每盒火柴数b降序
sort(v.rbegin(), v.rend());

long long ans = 0;
int rem = n;
for(auto &p : v)
{
    int b = p.first;
    int a = p.second;
    int take = min(a, rem);
    ans += 1LL * take * b;
    rem -= take;
    if(rem == 0) break;
}
cout << ans << endl;
return 0;

}