include

using namespace std; int a[5005], b[5005];

int main() {

int n, m;
scanf("%d%d", &n, &m);
for (int i = 1; i <= m ; i++) {
	scanf("%d%d", &a[i], &b[i]);
}
for (int i = 1; i <= m - 1; i++) {
	for (int j = 1; j <= m - i; j++) {
		if (a[j] > a[j + 1]) {
			int c, d;
			c = a[j];
			a[j] = a[j + 1];
			a[j + 1] = c;
			d = b[j];
			b[j] = b[j + 1];
			b[j + 1] = d;
		}
	}
}
int ans = 0;
for (int i = 1; i <= m; i++) {
	if (n <= 0) {
		break;
	}
	if (n - b[i] >= 0) {
		ans += a[i] * b[i];
		n -= b[i];
	} else {
		ans += a[i] * n;
		n = 0;
	}
}
printf("%d", ans);
return 0;

}//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇//牛浩宇