https://codeforces.com/problemset/problem/287/B
思路:贪心拿最大的。
每次取x个管子的式子为1+(2*k-x+1)*x/2-x,二分即可
#include<iostream>
#include<vector>
#include<queue>
#include<cstring>
#include<cmath>
#include<map>
#include<set>
#include<cstdio>
#include<algorithm>
#define debug(a) cout<<#a<<"="<<a<<endl;
using namespace std;
const int maxn=1e5;
typedef long long LL;
inline LL read(){LL x=0,f=1;char ch=getchar(); while (!isdigit(ch)){if (ch=='-') f=-1;ch=getchar();}while (isdigit(ch)){x=x*10+ch-48;ch=getchar();}
return x*f;}
bool check(LL x,LL k,LL n){
if(1+(2*k-x+1)*x/2-x>=n) return true;
else return false;
}
int main(void)
{
cin.tie(0);std::ios::sync_with_stdio(false);
LL n,k;cin>>n>>k;
if(n==1){
cout<<"0"<<"\n";
}
else{
LL l=1;LL r=k+1;
while(l<r){
LL mid=(l+r)>>1;
if( check(mid,k,n) ) r=mid;
else l=mid+1;
}
if(l==k+1){
cout<<"-1"<<"\n";
}
else cout<<l<<"\n";
}
return 0;
}