/* flight scheduling: minimum number of pairings */ min: x1+ x2+ x3+ x4+ x5+ x6+ x7+ x8+ x9+ x10+ x11+ x12 + x13 + x14 + x15 + x16 + x17 + x18 + x19 + x20 + x21 +x22; c1:x1+x2+x3+x4+x5+x6+x7+x8+x9 +x21 >= 1; c2:x1+x2+x3+x8+x9+x10+x11+x16+x17 >= 1; c3:x2+x4+x6+x7+x8+x9+x10+x12+x14+x15+x16+x17 +x19 +x20 >= 1; c4:x3+x5+x11+x13 >= 1; c5:x6+x8+x14+x16 +x19>= 1; c6:x7+x9+x15+x17+x18+x20 >= 1; c7:x4+x5+x6+x7+x12+x13+x14+x15+x21 >= 1; c8:x18+x22 >=1; int x1 x2 x3 x4 x5 x6 x7 x8 x9 x10 x11 x12 x13 x14 x15 x16 x17 x18 x19 x20 x21;