def count_trees(l, m, ranges): # 初始化所有树都在 trees = set(range(l + 1)) # 遍历每个区域 for start, end in ranges: # 移除区域内的树 trees -= set(range(start, end + 1)) # 返回剩余树的数量 return len(trees) # 读取输入 l, m = map(int, input().split()) ranges = [tuple(map(int, input().split())) for _ in range(m)] # 计算并输出结果 print(count_trees(l, m, ranges))