#include<stdio.h>

int main() {
    int l, m;
    scanf("%d %d", &l, &m);
    int a[100000];
    for (int i = 0; i < l; i++) {
        a[i] = 1;
        //1表示植树
    }
    int b[10000][2];
    for (int j = 0; j < m; j++) {
        scanf("%d %d", &b[j][0], &b[j][1]);
    }
    for (int k = 0; k < m; k++) {
        for (int n = b[k][0]; n < b[k][1]; n++) {
            a[n] = 0;
        }
    }
    int sum = 0;
    for (int o = 0; o < l; o++) {
        sum += a[o];
    }
    printf("%d", sum - 1);
    return 0;
}