CF787A - The Monster

 /*
  CF787A - The Monster
  http://codeforces.com/contest/787/problem/A
  数学 扩展欧几里得
  注意x或y为0的时候要特判
  并且结果要大于b和d
  */
 #include <cstdio>
 int ex_gcd(int a,int b,int &x,int &y)//solve x,y in a*x+b*y=ex_gcd(a,b,x,y)=gcd(a,b);
 {
     if(b==)
     {
         x=;
         y=;
         return a;
     }
     int ans=ex_gcd(b,a%b,x,y);
     int tmp=x;
     x=y;
     y=tmp-a/b*y;
     return ans;
     //x = x0 + (b/gcd)*t
     //y = y0 – (a/gcd)*t

 }
 int main()
 {
     int a,b,c,d;
     scanf("%d%d%d%d",&a,&b,&c,&d);
     int l=d-b;
     int x,y;
     if((b-d)%c== && (b-d)/c>= )//特判x==0时
     {
         printf("%d\n",b);
         return ;
     }
     if((d-b)%a== && (d-b)/a>= )//特判y==0时
     {
         printf("%d\n",d);
         return ;
     }
     int gcd=ex_gcd(a,c,x,y);
     if(l%gcd)
     {
         printf("-1\n");
         return ;
     }
     x*=l/gcd;
     c/=gcd;
     if(c<)
         c=-c;
     x%=c;
     while(x<= || b+x*a<d)//结果要大于b和d
         x+=c;
     printf("%d\n",b+x*a);
     return ;
 }