gistfile1.txt

def count_efficient_segments(n, m, pairs):
    not_collaborated = set()
    for x, y in pairs:
        if x > y:
            x, y = y, x
        not_collaborated.add((x, y))
    
    count = 0
    start = 1
    for end in range(1, n + 1):
        while start <= end:
            segment_is_valid = True
            for i in range(start, end):
                if (i, end) in not_collaborated:
                    segment_is_valid = False
                    break
            if segment_is_valid:
                count += 1
                break
            start += 1
    
    return count

# Read inputs from standard input
if __name__ == "__main__":
    n, m = map(int, input().split())
    pairs = []
    for _ in range(m):
        x, y = map(int, input().split())
        pairs.append((x, y))
    
    # Call the function and print the output
    result = count_efficient_segments(n, m, pairs)
    print(result)