#include <stdio.h>

int
main(void)
{
        int i;
        i = 0x3ffe+63; printf("%x\n", i);
        return 0;
}

% gcc47 -c ~/tmp/a.c
/home/kargl/tmp/a.c: In function 'main':
/home/kargl/tmp/a.c:7:6: error: invalid suffix "+63" on integer constant

+ is a binary operator. 0x3ffe is a hexidecimal constant and 63
is a decimal constant.