/* NOIP2005复赛 普及组 第2题 校门外的树(枚举) http://noi.openjudge.cn/ch0106/06/ https://www.luogu.org/problemnew/show/P1047 https://bbs.codeaha.com/problem-12002.html */ #include <iostream> using namespace std;

int road[10001];

int main() { int l, m, s, e, cnt = 0; cin >> l >> m;

for (int i = 0; i <= l; i++)
	road[i] = 1;

for (int i = 0; i < m; i++)
{
	cin >> s >> e;
	for (int j = s; j <= e; j++)  //start end
		road[j] = 0;
}

for (int i = 0; i <= l; i++)
	if (road[i] == 1)
		cnt++;

cout << cnt; 
return 0;

}