CodeForces - 785C

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
typedef long long int ll;
ll m,n;
int main()
{
    scanf("%lld%lld",&n,&m);
    if(m>=n)
        printf("%lld\n",n);
    else
    {
        ll cnt=n-m;
        ll l=1,r=3e9+7;
        while(l<=r)
        {   ll mid=(l+r)/2;
            if((mid+1)*mid/2>=cnt)//神奇的递加数列求和公式
                r=mid-1;
                else
                l=mid+1;
        }
        ll day=m+l;
        printf("%lld\n",day);
    }
    return 0;
}