#include <iostream>
using namespace std;

struct area
{
    int start;
    int end;
};

int main()
{
    int l_num[1000], l, m, many = 0;
    area m_num[100];
    cin >> l >> m;
    for (int i = 0; i < m; ++i)
    {
        cin >> m_num[i].start >> m_num[i].end;
    }
    for (int i = 0; i < l; ++i)
    {
        l_num[i] = 1;
    }
    for (int i = 0; i < m; ++i)
    {
        for (int j = m_num[i].start; j <= m_num[i].end; ++j)
        {
            l_num[j] = 0;
        }
    }
    for (int i = 0; i < l; ++i)
    {
        if (l_num[i] == 1)
        {
            ++many;
        }
    }
    cout << many;
    return 0;
}

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