#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
int main()
{
    int l, m;
    while(scanf("%d%d", &l, &m) != EOF)
    {
        int *a = (int *)malloc(sizeof(int)*(l+1));
        memset(a, 0, sizeof(int)*(l+1));
        int left, right, i;
        while(m--)
        {
            scanf("%d%d", &left, &right);
            for(i = left; i <= right; i++)
                ++a[i];
        }
        int count = 0;
        for(i = 0; i <= l; i++)
            if(!a[i])
                ++count;
        printf("%d\n", count);
        free(a);
    }
    //system("pause");
    return 0;
}
/**************************************************************
    Problem: 1088
    User: superlc320
    Language: C++
    Result: Accepted
    Time:40 ms
    Memory:1020 kb
****************************************************************/