初始list[0-500]=1 输入300 400的，则将list[300-400]=0 最后统计 1的个数即可

while True:
    try:
        l, m = map(int, input().strip().split())
        k = []
        s = 0
        for i in range(l + 1):
            k.append(1)
        for i in range(m):
            a, b = map(int, input().strip().split())
            for j in range(a, b + 1):
                if k[j] == 1: k[j] = 0

        for i in k:
            if i == 1: s += 1

        print(s)

    except:
        break