本质上就是gcd+高精度

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
using namespace std;
#define ll long long
inline int read()
{
    int x=0;bool t=false;char ch=getchar();
    while((ch<'0'||ch>'9')&&ch!='-')ch=getchar();
    if(ch=='-')t=true,ch=getchar();
    while(ch<='9'&&ch>='0')x=x*10+ch-48,ch=getchar();
    return t?-x:x;
}
const ll yw=1000000000000000000;
char ch[11000];
struct BigNum
{
    ll s[800];int ws;
    void output()
        {
            printf("%lld",s[ws]);
            for(int i=ws-1;i;--i)
                printf("%018lld",s[i]);
            puts("");
        }
    void clear(){memset(s,0,sizeof(s));ws=0;}
    void init(char *ch)
        {
            int l=strlen(ch+1);reverse(&ch[1],&ch[l+1]);
            for(int i=1;i<=l;i+=18)
            {
                ++ws;ll ss=1;
                for(int j=0;j<18;++j)
                    if(i+j<=l)s[ws]+=(ch[i+j]-48)*ss,ss*=10;
                    else break;
            }
        }
    void Div2()
        {
            for(int i=ws;i;--i)s[i-1]+=(s[i]&1)*yw,s[i]>>=1;s[0]=0;
            while(!s[ws])--ws;
        }
    void Multi2()
        {
            for(int i=1;i<=ws;++i)s[i]=s[i]<<1;
            for(int i=1;i<=ws;++i)s[i+1]+=s[i]/yw,s[i]%=yw;
            while(s[ws+1])++ws,s[ws+1]+=s[ws]/yw,s[ws]%=yw;
        }
}A,B,One,tmp;
BigNum operator-(BigNum a,BigNum b)
{
    int ws=max(a.ws,1);
    for(int i=1;i<=ws;++i)a.s[i]-=b.s[i];
    for(int i=ws-1;i;--i)if(a.s[i]<0)a.s[i]+=yw,a.s[i+1]-=1;
    while(!a.s[ws])--ws;
    a.ws=ws;return a;
}
bool operator<(BigNum a,BigNum b)
{
    if(a.ws!=b.ws)return a.ws<b.ws;
    for(int i=a.ws;i;--i)
        if(a.s[i]!=b.s[i])return a.s[i]<b.s[i];
    return false;
}int main()
{
    scanf("%s",ch+1);A.init(ch);
    scanf("%s",ch+1);B.init(ch);
    One.s[1]=One.ws=1;int two=0;
    if(B<A)swap(A,B);
    while(233)
    {
        if(A<One)break;
        if(!(A.s[1]&1)&&!(B.s[1]&1))A.Div2(),B.Div2(),++two;
        else if(!(A.s[1]&1))A.Div2();
        else if(!(B.s[1]&1))B.Div2();
        else B=B-A;if(B<A)swap(A,B);
    }
    while(two)B.Multi2(),--two;B.output();
    return 0;
}