差分

#include <iostream>
using namespace std;

const int N = 1e4 + 10;
int a[N], b[N];

void insert(int l, int r, int c)
{
    b[l] += c;
    if (r + 1 < N) b[r + 1] -= c;
}

int main()
{
    ios::sync_with_stdio(false);
    int n, m, sum = 0;
    cin >> n >> m;
    for (int i = 0; i <= n; i ++ ) a[i] = 1;
    for (int i = 0; i <= n; i ++ ) insert(i, i, a[i]);
    while (m -- ) 
    {
        int l, r;
        cin >> l >> r;
        insert(l, r, -1);
    }
    for (int i = 0; i <= n; i ++ )
    {
        b[i] += b[i - 1];
        if (b[i] > 0) ++ sum ;
    }
    cout << sum;
    cout << endl;
    return 0;
}