float U2 = 0; 
float R2 = 10000;
float U1=0 ;
float V=0 ;
void setup() {
  Serial.begin(9600);      
}
void loop() {

V = analogRead(A3);  

float U1 = 5*V/1024 ;

float U2 = 5-U1 ;

float R1 = R2 * U1/U2;

delay(500);

Serial.println(R1);

}