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RabbitStepping(SRM 475 DIV1 Easy)

import java.util.*;
import java.lang.*;
import java.math.*;
public class RabbitStepping {
 public double getExpected(String field, int r) {
  int n=field.length();
  int cnt=0, sum=0;
  for(int i=0; i<1<<n; i++){
   if(Integer.bitCount(i)==r){
    int r1=0, r2=0;
    for(int k=0; k<n; k++){
     if((i>>k&1)==1){
      if(k%2==0) r1++;
      else  r2++;
     }
    }
    cnt+=r1%2+r2%2;
    sum++;
   }
  }
  return (double)cnt/sum;
 }
}


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