#include <bits/stdc++.h>
using namespace std;
int n, m, d[1000001], p[1000001], m1[1000001], ma, c;

int main() {
	cin >> n >> m;
	for (int i = 1; i <= m; i++) {
		cin >> d[i] >> p[i];
		if (d[i] <= n) {
			m1[i] = p[i];
		}
	}
	for (int i = 1; i <= m; i++) {
		if (m1[i] > ma) {
			ma = m1[i];
		}
		c += ma;
		if (n >= p[i]) {
			n -= p[i];
		} else {
			break;
		}

		if (n <= 0) {
			cout << c;
			return 0;
		}
	}

	return 0;
}