#import <Foundation/Foundation.h>

int main()
{
    @autoreleasepool{
        NSData *stdinData = [[NSFileHandle fileHandleWithStandardInput] availableData];
        NSString *stdinString = [[NSString alloc] initWithData:stdinData encoding:NSUTF8StringEncoding];
		NSInteger R = [stdinString intValue];
		NSString *ans = [NSString stringWithFormat:@"%.2f\n", 2 * M_PI * R];

        NSFileHandle *fileHandle = [NSFileHandle fileHandleWithStandardOutput];
        [fileHandle writeData:[ans dataUsingEncoding:NSUTF8StringEncoding]];
        [fileHandle closeFile];
    }
}
