#include <stdio.h>
#include <string.h>
 
inline int ishex(int x)
{
	return	(x >= '0' && x <= '9')	||
		(x >= 'a' && x <= 'f')	||
		(x >= 'A' && x <= 'F');
}
 
int decode(const char *s, char *dec)
{
	char *o;
	const char *end = s + strlen(s);
	int c;
 
	for (o = dec; s <= end; o++) {
		c = *s++;
		if (c == '+') c = ' ';
		else if (c == '%' && (	!ishex(*s++)	||
					!ishex(*s++)	||
					!sscanf(s - 2, "%2x", &c)))
			return -1;
 
		if (dec) *o = c;
	}
 
	return o - dec;
}