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

float sigma3(float a, float b, float c);

int main() {
    float x, y, z, a;
    printf("Values: ");
    scanf("%g, %g, %g", &x, &y, &z);
    a = sigma3(x, y, z);
    printf("sigma = %g\n", a);
    return 0;
}

float sigma3(float a, float b, float c)
{
    float mean, s;
    mean = (a + b + c) / 3;
    s = pow(mean-a, 2)
	+ pow(mean-b, 2)
	+ pow(mean-c, 2);
    return sqrt(s/3);
}