题解 | #剩下的树#

# include<stdio.h>
# include<string.h>

const int N = 10005;

int buf[N];

int main()
{
	int L, M; 
	while(scanf("%d%d", &L, &M) != EOF){
		memset(buf, 0, sizeof(buf));
		while(M --){
			int left, right;
			scanf("%d%d", &left, &right);
			for(int i = left; i <= right; i ++){
				buf[i] = 1;
			}	
		}
		int res = 0;
		for(int i = 0; i <= L; i ++){
			if(buf[i] == 0) res ++;
		}
		printf("%d\n", res);
	}
	
	return 0;
}