float Uges   = 8.92;
float Rref   = 995000;
float RiMess = 998000;
float Ur     = 3.81;
float R;
static char disp[16];

void setup() {
  Serial.begin(115200);
  Serial.print("Ur= ");      Serial.println(Ur); 
  Serial.print("Uges = ");   Serial.println(Uges); 
  Serial.print("RiMess = "); Serial.println(RiMess); 
  
  R = (Ur * RiMess * Rref) / ( (RiMess * Uges) - (RiMess * Ur) - (Rref * Ur) );

  dtostrf( R, 5, 3, disp );

  Serial.print("R = "); Serial.println(disp); 

}

void loop() {
  
}