"""{ def perms(0) == 1
     def perms(n) == perms(n-1) * n  }"""

n = readInt()
i = 0
fac = 1
"""{ 1. perms(0) == 1       def
     2. fac == 1           premise
     3. fac == perms(0)     subst 1 2
}"""
while i != n :
    """{ invariant  fac == perms(i)
         modifies i, fac }""" 
    """{ 1. fac == perms(i)   premise   }"""
    i = i + 1
    """{ 1. i == i_old + 1          premise
         2. fac == perms(i_old)     premise  # from the invariant
         4. fac == perms(i-1)       algebra 1 2
    }"""
    fac = fac * i
    """{ 1. fac == fac_old * i           premise
         2. fac_old == perms(i-1)        premise
         3. fac == perms(i-1) * i        subst 2 1
         4. perms(i) == perms(i-1) * i   def
         5. fac == perms(i)              subst 4 3
    }"""

"""{ 1. not(i != n)       premise
     2. i == n            algebra 1
     3. fac == perms(i)   premise
     4. fac == perms(n)   subst 2 3
}"""