topological_sorting_1_source_removal

def get_pred_count(graph):
    result = {}
    for node, children in graph.items():
        if node not in result:
            result[node] = 0
        for child in children:
            if child not in result:
                result[child] = 0
            result[child] += 1
    return result


def find_node_with_zero_count(pred_counts):
    for node, count in pred_counts.items():
        if count == 0:
            return node
    return None


def top_sort(graph, pred_counts):
    removed_nodes = []
    while pred_counts:
        node = find_node_with_zero_count(pred_counts)
        if node is None:
            break

        removed_nodes.append(node)
        del pred_counts[node]
        for child in graph[node]:
            pred_counts[child] -= 1

    return removed_nodes


n = int(input())
graph = {}

for _ in range(n):
    line = input().split('->')
    if len(line) == 1 or line[1] == '':
        graph[line[0].strip()] = []
    else:
        graph[line[0].strip()] = line[1].strip().split(', ')

pred_counts = get_pred_count(graph)

result = top_sort(graph, pred_counts)

if pred_counts:
    print('Invalid topological sorting')
else:
    print(f'Topological sorting: {", ".join(result)}')