请计算n的阶乘(1<=n<=100)n的阶乘计算公式为:n!=nx(n-1)x(n-2)x...x1,如:5!=5x4x3x2x1=120
一个整数n(1<=n<=100)
n的阶乘
样例输入1
20
样例输出1
2432902008176640000