/in/foo.c: In function 'main':
/in/foo.c:7:10: warning: implicit declaration of function 'c' [-Wimplicit-function-declaration]
     cout=c(n,m);
          ^
/in/foo.c:8:10: warning: implicit declaration of function 'sum' [-Wimplicit-function-declaration]
     sout=sum(cout);
          ^~~
/in/foo.c: In function 'sum':
/in/foo.c:21:2: error: expected ',' or ';' before 'for'
  for(int i=0;i<k/2;i++)
  ^~~
/in/foo.c:21:14: error: 'i' undeclared (first use in this function)
  for(int i=0;i<k/2;i++)
              ^
/in/foo.c:21:14: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:21:23: error: expected ';' before ')' token
  for(int i=0;i<k/2;i++)
                       ^
/in/foo.c:21:23: error: expected statement before ')' token