/in/foo.c: In function 'main':
/in/foo.c:14:12: warning: implicit declaration of function 'sqrt' [-Wimplicit-function-declaration]
    int n = sqrt(m);
            ^~~~
/in/foo.c:14:12: warning: incompatible implicit declaration of built-in function 'sqrt'
/in/foo.c:14:12: note: include '<math.h>' or provide a declaration of 'sqrt'
/in/foo.c:15:4: warning: statement with no effect [-Wunused-value]
    for (a;a <= n;a++)
    ^~~