参考答案：

#include<iostream>
#include<vector>
using namespace std;
const int N = 100010;
int a[N];
int main()
{
    int n, m, sum=0;
    cin >> n >> m;
    while (m--)
    {
        int l, r;
        cin >> l >> r;
        if (l > r) swap(l, r);
        for (; l <= r; ++l)
        {
            a[l] = 1;
        }
    }
    for (int i = 1; i <= n; ++i)
    {
        if (a[i] != 1)
            ++sum;
    }
    cout << sum << endl;

    return 0;
}