class Station:
    def __init__(self, dist_from_a, cost, distancePerOneLitter):
        self.gaz_count = dist_from_a / distance_for_one_liter
        self.cost = cost

    def __str__(self):
        return "(" + str(self.gaz_count) + " " + str(self.cost) + ")"


def get_cost(cost_str):
    a, b = cost_str.split('.')
    b = int(b)
    if b < 10:
        b *= 10

    return int(a) * 100 + b


if __name__ == "__main__":
    distance = int(input())

    nums = input().split(" ")
    capacity = int(nums[0])
    distance_for_one_liter = int(nums[1])
    money_spend_in_A = get_cost(nums[2])
    stations_count = int(nums[3])

    stations = []
    stations.append(Station(0, 0, distance_for_one_liter))
    for i in range(stations_count):
        nums = input().split(" ")
        dist_from_a = int(nums[0])
        cost = get_cost(nums[1])
        stations.append(Station(dist_from_a, cost, distance_for_one_liter))

    d = []
    d.append(0)
    for i in range(1, len(stations)):
        costs = []
        all_costs = []
        for j in range(len(d)):
            if float(capacity) / 2 - 0.001 <= stations[i].gaz_count - stations[j].gaz_count <= capacity + 0.001:
                costs.append(d[j] + 2000 + (stations[i].gaz_count - stations[j].gaz_count) * stations[i].cost)
            if stations[i].gaz_count - stations[j].gaz_count <= capacity + 0.001:
                all_costs.append(d[j] + 2000 + (stations[i].gaz_count - stations[j].gaz_count) * stations[i].cost)
        if len(costs) > 0:
            d.append(sorted(costs)[0])
        else:
            d.append(sorted(all_costs)[0])

    gaz_to_end = distance / distance_for_one_liter


    station_ids_to_go_to_end = []
    for stationId in range(len(stations)):
        if stations[stationId].gaz_count > gaz_to_end - capacity - 0.0001:
            station_ids_to_go_to_end.append(stationId)

    costs_to_end = []
    for station_to_end_id in station_ids_to_go_to_end:
        costs_to_end.append(d[station_to_end_id])

    intValue = str(int(sorted(costs_to_end)[0] + money_spend_in_A))
    print(intValue[:-2], ".", intValue[-2:], sep="")