z1

#include <stdio.h>
#include <math.h>

int main(void){
	FILE *test;
	long long a;
	int code;
	long long sum1 = 0, sum2 = 0;
	long double ans;
	int count = 0;
	if((test = fopen("test.txt","r")) == NULL){
		printf("Can't open file!\n");
		getchar();
		return 1;
	}
	while((code=fscanf(test,"%lld",&a)) == 1){
		sum1 += a*a;
		sum2 += a;
		count++;
	}
	if(code != EOF){
		printf("Can't read data from file!");
		getchar();
		fclose(test);
		return 2;
	}
	if(count == 0){
		printf("File is empty!");
		getchar();
		fclose(test);
		return 2;
	}
	ans = (double)(sum1*count - sum2*sum2)/(count*count);
	printf("%Lf\n",ans);
	fclose(test);
	return 0;
}