#include <cctype>
#include <cstdio>
#include <algorithm>
using namespace std;
int n, m, t, ans;
struct node {
	int p, x;
} milk[2000005];
inline int read() { // 快读
	int n = 0;
	char ch = getchar();
	while (!isdigit(ch))
		ch = getchar();
	while (isdigit(ch)) {
		n = (n << 3) + (n << 1) + (ch - 48);
		ch = getchar();
	}
	return n;
}
bool cmp(node a, node b) { // 贪心，按单价升序排序
	return a.p < b.p;
}
int main() {
	freopen("milk.in", "r", stdin);
	freopen("milk.out", "w", stdout); // 文件输入输出
	n = read(), m = read();
	for (int i = 0; i < m; i++)
		milk[i].p = read(), milk[i].x = read();
	sort(milk, milk + m, cmp); // 排序
	for (int i = 0; i < m && n > 0; i++) {
		if (n >= milk[i].x) t = milk[i].x, n -= t;
		else t = n, n = 0;
		ans += t * milk[i].p;
	}
	printf("%d", ans);
	return 0;
}