Strange Expression Result

In C,
when I have, code lines as

int i,c ;
i = 5 ;
c = ++i + ++i ;

it produces output(value of c) as 14, and when I have code lines as

int i=5 ;
int c = ++i + ++i ;

it produces output(value of c) as 13. What's the difference between these two codes and what's really happening in there?