Dirichlet's Theorem on Arithmetic Progressions

http://poj.org/problem?id=3006

 #include<stdio.h>
 #include<math.h>
 int is_prime(int n)
 {
     int i,m;
     if (n==)
         return ;
     m = sqrt(n);
     for (i = ; i <= m; i ++)
     {
         if (n%i==)
             return ;
     }
     return ;
 }
 int main()
 {
     int a,d,n,i;
     while(~scanf("%d%d%d",&a,&d,&n))
     {
         int cnt = ;
         if (a==&&d==&&n==)
             break;
         for (i = a;; i =i+d)
         {
             if(is_prime(i))
                 cnt++;
             if(cnt==n)
                 break;
         }
         printf("%d\n",i);
     }
     return ;
 }