C++

using namespace std;
#define MAX 10000
bool L[MAX + 1];//true代表有树，false没有树
int main() {
	int l, M;
	while (cin >> l >> M) {
		for (int i = 0; i <= l; i++) {
			L[i] = true;//初始有树
		}
		for (int i = 1; i <= M; i++) {
			int x1, x2;
			cin >> x1 >> x2;
			for (int j = x1; j <= x2; j++) {
				L[j] = false;
			}
		}
		int sum = 0;
		for (int i = 0; i <= l; i++) {
			sum += (L[i]) ? 1 : 0;
		}
		cout << sum << endl;
	}

	return 0;
}