min   700y13 + 400y14 + 450y25 + 600y56 + 300y57
    + 500y58 + 500y68 + 400y78 
subject to
2)  x2 - x1       >=  5
3)  x3 - x1 + y13 >= 10
4)  x4 - x1 + y14 >= 11
5)  x5 - x2 + y25 >= 4
6)  x4 - x3       >= 4
7)  x5 - x3       >= 0
8)  x6 - x4       >= 15
9)  x6 - x5 + y56 >= 21
10) x7 - x5 + y57 >= 25
11) x8 - x5 + y58 >= 35
12) x7 - x6       >= 0
13) x8 - x6 + y68 >= 20
14) x8 - x7 + y78 >= 15
15) x8 - x1 <= 49
end
sub y13 2
sub y14 3 
sub y25 1
sub y56 5
sub y57 3
sub y58 5
sub y68 4
sub y78 3