n, k= list(map(int,str.split(input())))
max_pos=(((k-1)*k)//2)+1
if n==1:
print(0)
elif n<=k:
print(1)
elif n>max_pos:
print(-1)
else:
m=((1+(1-(4*(n-max_pos)*2))**0.5)/2)
if m*(m-1)+n>max_pos:
m-=1
print(int(k-m))// Online C++ compiler to run C++ program online
#include <bits/stdc++.h>
using namespace std;
int main() {
long long n,k;
cin>>n>>k;
long long low=0,high=k;
long long maxpipes=(k*(k-1))/2+1;
while(low<=high){
long long mid=low+(high-low)/2;
long long poss=(k*mid)-((mid*(mid+1))/2)+1;
if(poss<n)low=mid+1;
else high=mid-1;
}
if(low>k){
cout<<-1;
}else cout<<low;
return 0;
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