Ошибка: IndexError: list index out of range

a = []
for i in range(100):
    a.append([0] * 100)

for i in range(1, 100):
    for j in range(1, 100):
        if i + (i + j) >= 80 or (i + j) + j >= 80:
            a[i][j] = 0

for n in range(150):
    for i in range(1, 100):
        for j in range(1, 100):
            if a[i][j] == 0:
                if a[i][i + j] < 0 or a[i + j][j] < 0:
                    a[i][j] = -1 * min(a[i][i + j], a[i + j][j]) + 1
                if a[i][i + j] > 0 and a[i + j][j] > 0:
                    a[i][j] = -1 * max(a[i][i + j], a[i + j][j])

for i in range(1, 50):
    if a[20][j] == 2:
        print(j)