def install_lightbulbs(M, R, street_lights):
    street_lights.sort()
    n = len(street_lights)
    i = 0
    num_bulbs = 0
    rightmost_illuminated = -1
    
    while i < n:
        if street_lights[i] - R > rightmost_illuminated:
            return -1 # there is a gap that cannot be illuminated
        leftmost_possible = street_lights[i]
        while i < n and street_lights[i] - R <= leftmost_possible:
            i += 1
        rightmost_illuminated = min(M, leftmost_possible + 2*R)
        num_bulbs += 1
        
    return num_bulbs