#include <stdio.h>
#include <stdlib.h>

int main() {
    FILE *fin, *fout;
    int m, n, i, max, maxdr, maxst, d, l;
    fin = fopen( "orase.in", "r" );
    fout = fopen( "orase.out", "w" );
    fscanf( fin, "%d%d", &m, &n );
    maxst = 0;
    maxdr = 0;
    for ( i = 0; i < n; i++ ) {
        fscanf( fin, "%d%d", &d, &l );
        if ( d + l > maxst )
            maxst = d + l;
        if ( ( m - d ) + l > maxdr )
            maxdr = ( m - d ) + l;
    }
    fprintf( fout, "%d", maxdr + maxst - m );
    fclose( fin );
    fclose( fout );
    return 0;
}