请问，如何添加一个约束条件为平均速度为每小时25公里？。

mod文件：

param n;
set ARCS := {i in 1..n, j in 1..n: i<>j}; 
param distance {ARCS} >=0; 
var Route {ARCS} binary; 
var Load {2..n} >=1, <= n-1; 
minimize Total_distance: sum {(i,j) in ARCS} distance [i,j] * Route[i,j];
subject to Out_Of {i in 1..n}:  sum {(i,j) in ARCS} Route[i,j] = 1;
subject to Route_Continuity {i in 1..n}:sum {(i,j) in ARCS} Route[i,j]= sum {(j,i) in ARCS} Route[j,i];
subject to Load_Continuity{i in 2..n, j in 2..n: i<>j}: Load[i] - Load[j] +1 - n*(1-Route[i,j]) <=0;

dat文件：

param n := 9;
param distance:
   1     2     3     4     5     6      7      8      9  :=
1  .     4000  3600  3700  4600  3200   3500   4200   1400
2  3300  .     1600  2300  1500  400    2800   3300   1500
3  3800  1600  .     1000  200   1300   2500   5100   2800
4  3500  2100  700   .     900   1600   3600   4200   2400
5  3900  2100  300   1100   .    1200   3100   4900   1200
6  1700  400   1500  1700  1200  .      2500   4600   2100
7  2800  1800  3200  3900  3200  2000   .      2700   3100
8  4700  4100  4800  5300  4300  4300   2800   .      4400
9  1300  1500  1800  2300  2100  1300   2500   3400   .   ;