/in/foo.c: In function 'main':
/in/foo.c:10:10: warning: right-hand operand of comma expression has no effect [-Wunused-value]
   for(i=3,i<=n;i++)
          ^
/in/foo.c:10:19: error: expected ';' before ')' token
   for(i=3,i<=n;i++)
                   ^
/in/foo.c:15:4: error: 's' undeclared (first use in this function)
    s=s+c;
    ^
/in/foo.c:15:4: note: each undeclared identifier is reported only once for each function it appears in