#include <stdio.h>
int main(void) {
    float radius;
    if (scanf("%f", &radius) != 1) {
        printf("Invalid input");
        return 0;
    }
    if (radius <= 0) {
        printf("Invalid input");
        return 0;
    }
    float circumference = 2.0f * 3.14159f * radius;
    printf(".2f", circumference);
    return 0;
}