foo.c:3:1: error: unknown type name 'using'
 using namespace std;
 ^
foo.c:3:17: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'std'
 using namespace std;
                 ^
foo.c: In function 'main':
foo.c:6:15: warning: unused variable 'k' [-Wunused-variable]
 int t,m,n,i,j,k,l,o;
               ^
foo.c:6:9: warning: unused variable 'n' [-Wunused-variable]
 int t,m,n,i,j,k,l,o;
         ^
foo.c:6:5: warning: variable 't' set but not used [-Wunused-but-set-variable]
 int t,m,n,i,j,k,l,o;
     ^