// editor3

#include <stdio.h>

int main() {
    float radius;
    
    // Read radius
    if (scanf("%f", &radius) !=1 {
        printf("Invalid input");
        return 0;
    }
    
    // Check for negative radius
    if (radius < 0) {
        printf("Invalid input");
        return 0:
    }
    
    // Calculate circumference
    float circumference = 2 * 3.14159 * radius;
    
    // Print result
    printf("%.2f", circumference);
    
    return 0;
}