Codeforces Round #404 (Div. 2) C. Anton and Fairy Tale

• Time : $O(log(INF))$
• Space : $O(1)$

def func(n):
    return n * (n + 1) // 2;
 
def helper(n, m, mid):
    k = n + (mid - m) * m;
    eat = func(mid) - func(m - 1);
    return k - eat;
 
def solve(n, m):
    if n <= m:
        return n;
    l = m
    r = 10**18
    res = 10**18;
    while l <= r :
        mid = l + (r - l) // 2;
        s = helper(n,m,mid);
        if s <= 0:
            res = min(res, mid);
            r = mid - 1;
        else:
            l = mid + 1;
 
    return res;
 
n, m = map(int,input().split())
 
print(solve(n,m))