import sys

def read_input():
    return list(map(int, sys.stdin.read().split()))

# Tries to reach the destination
# - if destination reachable, returns number of stops
# - otherwise: returns -1
def drive(distance, tank_capacity, stops):
    tank = tank_capacity
    distance_travelled = 0
    stops_made = 0

    if tank_capacity < distance - stops[-1]:
        fail(-1)
    # For every stop we are about to move to
    for stop in stops:
        if tank >= stop - distance_travelled:  # if we have enough fuel to reach the next stop
            tank, distance_travelled = move(stop, tank, distance_travelled)
            # print("Move: tank: {}, distance_travelled: {}".format(tank, distance_travelled))
            continue
        else:
            if tank_capacity >= stop - distance_travelled:  # if refilling will help us reach the next stop
                # print("Refilling: stop: {}, distance_travelled: {}".format(stop, distance_travelled))
                tank = tank_capacity  # refill
                stops_made += 1
                tank, distance_travelled = move(stop, tank, distance_travelled)
            else:
                fail(stop)
    if tank == 0 and distance != stop: # if our tank is empty and we need to move
        stops_made += 1 # refill
    return stops_made

def move(stop, tank, distance_travelled):
    distance_to_travel = stop - distance_travelled
    return (tank - distance_to_travel, stop)

def fail(stop):
    print("Give up: stop: {}".format(stop))
    return -1

if __name__ == "__main__":
    data = read_input()
    # data[2] - skipping, we don't care about the number of stops
    distance, tank_capacity, stops = data[0],  data[1], data[3:]
    stops_made = drive(distance, tank_capacity, stops)
    print(stops_made)