POJ 2115 线性模方程

#include <iostream>#include <cstdio>#include <cstdlib>#include <cstring>#include <algorithm>#include <vector>#include <string>#include <cmath>#include <stack>#include <queue>using namespace std;#define ll long long ll A,B,C,k,a,b;ll ext_gcd(ll a,ll n,ll &x,ll &y){if(n==0){x=1;y=0;return a;}ll d=ext_gcd(n,a%n,x,y);ll x1=x,y1=y;x=y1;y=x1-y1*(a/n);return d;}int main (){while(scanf("%lld%lld%lld%lld",&A,&B,&C,&k)!=EOF){if(A==0 && B==0 && C==0 && k==0) break;ll n=1,x,y;for(int i=1;i<=k;++i)n=n*2;b=B-A;while(b<0)b+=n;a=C;ll d=ext_gcd(a,n,x,y);if(b%d)printf("FOREVER\n");else{ll e=x*(b/d);if(e<0){if(e%(n/d)==0)e=0;else e+=(-e/(n/d)+1)*(n/d);}else{if(e%(n/d)==0)e=0;else e-=e/(n/d)*(n/d);}printf("%lld\n",e);}}return 0;}