1.3.1Mixing Milk

/*
ID: awsd1231
PROG: milk
LANG: C++
*/
#include<iostream>
#include<cstdio>
#include<map>
#include<algorithm>
using namespace std;
const int maxn =5000 + 10;
int n, m;
struct Pa {
	int p;
	int pa;
};
Pa pa[maxn];
bool operator < ( Pa a, Pa b) {
	if(a.p < b.p) return true;
	else return false;
}
int main() {
	freopen("milk.in", "r", stdin);
	freopen("milk.out", "w", stdout);
	cin >> n >> m;
	for(int i = 0; i != m; ++i) 
		cin >> pa[i].p >> pa[i].pa;
	sort(pa, pa+m);
	int need = n, cost = 0, idx = 0;
	while(need > 0) {
		cost += pa[idx].p * pa[idx].pa;
		need -= pa[idx++].pa;
	}
	cost -= pa[--idx].p * pa[idx].pa;
	need += pa[idx].pa;
	while(need--)
	   	cost += pa[idx].p;
	cout << cost << endl;
	return 0;
}