INF = 10 ** 9
MOD = 10 **9 + 7
import sys
sys.setrecursionlimit(100000000)
dy = (-1,0,1,0)
dx = (0,1,0,-1)

def main():
    n = int(input())
    print(3*n//2) 
if __name__=='__main__':
    main()