#include <Servo.h>
double ch;
int ch1 = 2000;

void setup() {
Serial.begin(9600);
}

void loop() {
  ch = map(ch1, 2000, 1750, 0,50 );
  double xch = ch - 3;
   Serial.print(xch);Serial.println("   ");
   //Serial.println(xch);
}