#include<stdio.h>

int main() {
	int n, m, package = 1000, each = 1000;
	scanf("%d %d", &n, &m);
	for (int i = 0; i < m; i++) {
		int p, e;
		scanf("%d %d", &p, &e);
		package = (p < package) ? p : package;
		each = (e < each) ? e : each;
	}
	if (n % 6 * each >= package) {
		printf("%d", package * (n / 6) + package);
	}
	else if (each * 6 < package) {
		printf("%d", n * each);
	}
	else {
		printf("%d", (n / 6) * package + n % 6 * each);
	}
    	return 0;
}