扩展欧几里得

LL extgcd(LL a,LL b,LL &x,LL &y){
	LL ret = a;
	if(b){
		ret = extgcd(b,a%b,y,x);
		y -= (a/b)*x;
	}else{
		x = 1;
		y = 0;
	}
	return ret;
}