1 // fft模板 HDU 1402
2
3 #include <iostream>
4 #include <cstdio>
5 #include <cstdlib>
6 #include <algorithm>
7 #include <vector>
8 #include <math.h>
9 #include <memory.h>
10 #include <bits/stdc++.h>
11 using namespace std;
12 #define LL long long
13 typedef pair<int,int> pii;
14 const LL inf = 0x3f3f3f3f;
15 const LL MOD =100000000LL;
16 const int N = 150010;
17 const double eps = 1e-8;
18 void fre() {freopen("in.txt","r",stdin);}
19 void freout() {freopen("out.txt","w",stdout);}
20 inline int read() {int x=0,f=1;char ch=getchar();while(ch>'9'||ch<'0') {if(ch=='-') f=-1; ch=getchar();}while(ch>='0'&&ch<='9') {x=x*10+ch-'0';ch=getchar();}return x*f;}
21
22
23 const double PI = acos(-1.0);
24 //复数结构体
25 struct Complex{
26 double r,i;
27 Complex(double _r = 0.0,double _i = 0.0){
28 r = _r; i = _i;
29 }
30 Complex operator +(const Complex &b){
31 return Complex(r+b.r,i+b.i);
32 }
33 Complex operator -(const Complex &b){
34 return Complex(r-b.r,i-b.i);
35 }
36 Complex operator *(const Complex &b){
37 return Complex(r*b.r-i*b.i,r*b.i+i*b.r);
38 }
39 };
40 /*
41 * 进行FFT和IFFT前的反转变换。
42 * 位置i和 （i二进制反转后位置）互换
43 * len必须去2的幂
44 */
45 void change(Complex y[],int len){
46 int i,j,k;
47 for(i = 1, j = len/2;i < len-1; i++){
48 if(i < j)swap(y[i],y[j]);
49 //交换互为小标反转的元素，i<j保证交换一次
50 //i做正常的+1，j左反转类型的+1,始终保持i和j是反转的
51 k = len/2;
52 while( j >= k){
53 j -= k;
54 k /= 2;
55 }
56 if(j < k) j += k;
57 }
58 }
59 /*
60 * 做FFT
61 * len必须为2^k形式，
62 * on==1时是DFT，on==-1时是IDFT
63 */
64 void fft(Complex y[],int len,int on){
65 change(y,len);
66 for(int h = 2; h <= len; h <<= 1){
67 Complex wn(cos(-on*2*PI/h),sin(-on*2*PI/h));
68 for(int j = 0;j < len;j+=h){
69 Complex w(1,0);
70 for(int k = j;k < j+h/2;k++){
71 Complex u = y[k];
72 Complex t = w*y[k+h/2];
73 y[k] = u+t;
74 y[k+h/2] = u-t;
75 w = w*wn;
76 }
77 }
78 }
79 if(on == -1)
80 for(int i = 0;i < len;i++)
81 y[i].r /= len;
82 }
83
84 Complex a[N],b[N];
85 char s1[N/2],s2[N/2];
86 int ans[N];
87 int main(){
88 while(~scanf("%s%s",s1,s2)){
89 int len1=strlen(s1);
90 int len2=strlen(s2);
91 int l1=0,l2=0;
92 while((1<<l1)<len1) l1++;
93 while((1<<l2)<len2) l2++;
94 int len=(1<<(max(l1,l2)+1));
95 for(int i=0;i<len;i++){
96 if(i<len1) a[i]=Complex(s1[len1-i-1]-'0',0);
97 else a[i]=Complex(0,0);
98 if(i<len2) b[i]=Complex(s2[len2-i-1]-'0',0);
99 else b[i]=Complex(0,0);
100 }
101 fft(a,len,1);
102 fft(b,len,1);
103 for(int i=0;i<len;i++)
104 a[i]=a[i]*b[i];
105 fft(a,len,-1);
106 for(int i=0;i<len;i++)
107 ans[i]=(int)(a[i].r+0.5);
108 for(int i=0;i<len-1;i++){
109 ans[i+1]+=ans[i]/10;
110 ans[i]%=10;
111 }
112 bool flag=false;
113 for(int i=len-1;i>=0;i--){
114 if(ans[i]) printf("%d",ans[i]),flag=true;
115 else if(flag||i==0)printf("0");
116 }
117 printf("\n");
118 }
119 return 0;
120 }
