View Compilation Error

0_0_30602089_12307.cpp:5:1: error: 'db' does not name a type
 db c(db a) {
 ^
0_0_30602089_12307.cpp:12:1: error: 'db' does not name a type
 db f(db n, db i) {
 ^
0_0_30602089_12307.cpp: In function 'int main()':
0_0_30602089_12307.cpp:16:3: error: 'db' was not declared in this scope
   db n, i, sum;
   ^
0_0_30602089_12307.cpp:17:25: error: 'n' was not declared in this scope
   while (~scanf("%lf", &n)) {
                         ^
0_0_30602089_12307.cpp:18:5: error: 'sum' was not declared in this scope
     sum = 0;
     ^
0_0_30602089_12307.cpp:19:10: error: 'i' was not declared in this scope
     for (i = 2; i <= n; i++)
          ^
0_0_30602089_12307.cpp:20:26: error: 'f' was not declared in this scope
       sum += (i-1)*f(n, i); // 排好序的 总共有n个数字，从中选取m个数字（n>=m>=2）, 在m个数中插板，总共有m-1种可能
                          ^