minL=x11+2x12+4x13+x14+2x21+3x22+x23+5x24+3x31+2x32+4x33+4x34
x11+x12+x13+x14=50
x21+x22+x23+x24=30
x31+x32+x33+x34=10
x11+x21+x31=30
x12+x22+x32=30
x13+x23+x33=10
x14+x24+x34=20