$ ranges -N=7 -M=9
 0 : from       0 to       1 (      2)
 1 : from       2 to       3 (      2)
 2 : from       4 to       5 (      2)
 3 : from       6 to       7 (      2)
 4 : from       8 to       9 (      2)
 5 : from      10 to      11 (      2)
 6 : from      12 to       9 (     -2)