cf B. Making Sequences is Fun

http://codeforces.com/contest/373/problem/B

用二分枚举长度就可以。

 #include <cstdio>
 #include <cstring>
 #include <algorithm>
 #define LL __int64
 using namespace std;

 LL w,m,k;

 bool ok(LL c)
 {
     LL ans=;
     LL x2=m+c-;
     int t1=;
     LL b=x2;
     while(b)
     {
         b/=;
         t1++;
     }
     int t2=;
     LL b1=m;
     while(b1)
     {
         b1/=;
         t2++;
     }
     if(t1==t2)
     {
         ans+=c*t1*k;
         if(ans<=w&&ans>=) return true;
         else return false;
     }
     LL x1=;
     int a1=t1-;
     while(a1--)
     {
         x1*=;
     }
     ans+=(x2-x1+)*t1*k;
     LL x3=;
     int a2=t2;
     while(a2--)
     {
         x3*=;
     }
     ans+=(x3-m)*t2*k;
     for(LL i=x3; i<x1; i*=)
     {
         t2++;
         ans+=(i*-i)*t2*k;
     }
     if(ans<=w&&ans>=) return true;
     else return false;

 }

 int main()
 {
     while(scanf("%I64d%I64d%I64d",&w,&m,&k)!=EOF)
     {
         LL l=,r=w;
         LL ans=;
         while(l<=r)
         {
             LL mid=(l+r)/;
             if(ok(mid))
             {
                 ans=mid;
                 l=mid+;
             }
             else
                 r=mid-;
         }
         printf("%I64d\n",ans);
     }
     return ;
 }