python プログラムを終了

すみません、以下のコードで
もし"A"がheight+1かwidth+1にプリントされた時にプログラムが終了する様にするにはどうすれば良いでしょうか？

python
1import random
2import time
3
4# Ask the user to input how big the grid is.
5width = int(input("State the width of the grid: "))
6height = int(input("State the height of the grid: "))
7
8ant = ["A"]
9grid = {}
10previouspos = (-1, -1)
11
12# find the empty space in the grid.
13for y in range(height+1):
14    for x in range(width+1):
15        coordinate = (x, y)
16        grid[coordinate] = " "
17
18# Print the grid.
19def print_grid():
20    for y in range(height):
21        for i in range(width):
22            print("+---", end="")
23        print("+")
24        for x in range(width):
25            if x == 0:
26                print("|", end="")
27            coordinate = (x, y)
28            print("", grid[coordinate], end=" |")
29        if y != height:
30            print("")
31    for i in range(width):
32        print("+---", end="")
33    print("+")
34
35def change_grid():
36    # Place ant in the grid. Randomly.
37    place, ant_place = random.choice(list(grid.items()))
38    ant_place = random.choice(ant)
39    grid[place] = ant_place
40
41
42# Printing the grid and changing it every 1 seconds
43for turns in range(10):
44    print_grid()
45    time.sleep(1)
46    for y in range(height):
47        for x in range(width):
48            coordinate = (x, y)
49            if (previouspos == coordinate):
50                grid[coordinate] = " "
51    place, item = random.choice(list(grid.items()))
52    ant_place = random.choice(ant)
53    grid[place] = ant_place
54    previouspos = place
55    if turns != 4:
56        print()
57
58
59
60print_grid()
61