#include <stdio.h>
int main(){
int N;
double sum=0.0;
int i;
int s=1;
if (scanf("%d",&N)!=EOF){
for (i=0;i<N;i++){
sum+=s*1.0/(1+3*i);
s*=-1;
}
printf("sum = %.3f",sum);
}else{
printf("int N wrong");
}
return 0;
}
京公网安备 11010702002337 号