rt

#include<iostream>
#define ll long long
using namespace std;
ll n, l, a[100001][3], ans = 21474836477777;
int main() {
	cin >> l >> n;
	for (ll i = 1; i <= n; i++) {
		cin >> a[i][1] >> a[i][2];
	}
	for (ll i = 1; i <= l; i++) {
		ll minn = 0;
		for (ll j = 1; j <= n; j++) {
			minn += (max(i, a[j][1]) - min(i, a[j][1])) * a[j][2];
		}
		ans = min(ans, minn);
	}
	cout << ans;
	return 0;
}