#include <stdio.h>

int main (){
    FILE *fin, *fout;
    int c, n, i, a1, a2, h1, m1, h2, m2, maxim = 0;
    fin = fopen ("aeriana.in", "r");
    fscanf (fin, "%d%d", &c, &n);
    if (c == 1){
        for (i = 0; i < n; i ++){
            fscanf (fin, "%d%d%d%d%d%d", &a1, &a2, &h1, &m1, &h2, &m2);
            if (h1 > h2 || (h1 == h2 && m1 >= m2))
                h2 += 24;
            if (h2 * 60 + m2 - h1 * 60 - m1)
                maxim = h2 * 60 + m2 - h1 * 60 - m1;
        }
    }
    fclose (fin);
    fout = fopen ("aeriana.out", "w");
    fprintf (fout, "%d\n", maxim);
    fclose (fout);
    return 0;
}