#include <stdio.h>
int main(){
int N;
double sum=0.0;
int i;
if (scanf("%d",&N)!=EOF){
for (i=0;i<N;i++){
sum+=1.0/(2*i+1);
}
printf("sum = %.6f",sum);
}else{
printf("int N wrong");
}
return 0;
}
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