foo.c: In function 'main':
foo.c:18:3: error: 'for' loop initial declarations are only allowed in C99 mode
   for(int i=0;i<=c;i+=2){
   ^
foo.c:18:3: note: use option -std=c99 or -std=gnu99 to compile your code
foo.c:29:3: error: 'for' loop initial declarations are only allowed in C99 mode
   for(int i=1;i<=c;i+=2){
   ^