/in/foo.c: In function 'Concat':
/in/foo.c:21:18: warning: format '%s' expects argument of type 'char *', but argument 2 has type 'int' [-Wformat=]
         printf("%s",s2[i]);
                  ^
/in/foo.c:5:9: warning: unused variable 'diff' [-Wunused-variable]
     int diff = 0;
         ^~~~