题目链接:https://blog.csdn.net/shengtao96/article/details/51234355
AC代码:
#include<iostream>
#include<cmath>
# define INF 0x7fffffff
# define ll long long
using namespace std;
ll exgcd(ll a,ll b ,ll &x,ll &y)
{
if(!b)
{
x=1;
y=0;
return a;
}
ll ans=exgcd(b,a%b,x,y);
ll t=x;
x=y;
y=t-a/b*y;
return ans;
}
int main()
{
ll A,B,C,k;
while(cin>>A>>B>>C>>k&&A+B+C+k)
{
ll b=(ll)1<<k;
ll x,y;
ll gcd=exgcd(C,b,x,y),cc=B-A;
if(cc%gcd==0)
{
x*=cc/gcd;
b/=gcd;
x=(x%b+b)%b;
cout<<x<<endl;
}
else cout<<"FOREVER"<<endl;
}
return 0;
}