数据太水了，纯暴力都过了

#include <cstdio>
#include <map>
#include <algorithm>
using namespace std;

const int maxn = 10000005;
const int maxm = 2005;
int n, m;
int cnt, maxr, ans;
int wall[maxn << 2];
map<int, bool> appeared;
struct Paper {
    int left, right;
} paper[maxm];

int main() {
    scanf("%d%d", &n, &m);
    for (int i = 1; i <= m; i++) {
        scanf("%d%d", &paper[i].left, &paper[i].right);
        maxr = max(maxr, paper[i].right);
    }
    for (int i = 1; i <= m; i++) {
        for (int j = paper[i].left; j <= paper[i].right; j++) {
            wall[j] = i;
        }
    }
    for (int i = 1; i <= maxr; i++) {
        if (!appeared[wall[i]] && wall[i] != 0) {
            appeared[wall[i]] = true;
            ans++;
        }
    }
    printf("%d", ans);
    return 0;
}