How Do I Solve The Following Integer Programming Problem By Using Branch And Bou

How do i solve the following integer programming problem by using branch and bound?

minimize x1+x2+x3+x4+x5+x6

subject to:

x1+x2>=7

x2+x3>=6

x3+x4>=6

x4+x5>=5

x5+x6>=4

x6+x1>=4

x1, x2, x3, x4, x5, x6 >=0