From the linked code (the compiler won't complain about this pile of pooh)
if(Serial.available()>0)
{
x[0]=Serial.read();
x[1]=Serial.read();
x[2]=Serial.read();
x[3]=Serial.read();
x[4]=Serial.read();
SUM=x[0]+x[1]+x[2]+x[3];
if(SUM==x[4])