Problem:
Score : 300 points

Problem Statement

Snuke loves puzzles.

Today, he is working on a puzzle using S- and c-shaped pieces.
In this puzzle, you can combine two c-shaped pieces into one S-shaped piece, as shown in the figure below:

Snuke decided to create as many Scc groups as possible by putting together one S-shaped piece and two c-shaped pieces.

Find the maximum number of Scc groups that can be created when Snuke has N S-shaped pieces and M c-shaped pieces.

Constraints

1 ≤ N,M ≤ 10^{12}

Input

The input is given from Standard Input in the following format:

N M

Output

Print the answer.

Sample Input 1

1 6

Sample Output 1

2

Two Scc groups can be created as follows:

Combine two c-shaped pieces into one S-shaped piece

Create two Scc groups, each from one S-shaped piece and two c-shaped pieces

Sample Input 2

12345 678901

Sample Output 2

175897

Solution in C (Clang 3.8.0) (judge verdict: Wrong Answer)

#include <stdio.h>

int main() {
    unsigned long long int n, m;
    scanf("%lld %lld", &n, &m);

    unsigned long long int r = 0;
    if (n*2 <= m) {
        r += n;
        m -= n*2;
        n = 0;
    } else if (n <= m/2) {
        r += m/2;
        n -= m/2;
        m = 0;
    }

    if (m >= 4) {
        r += m / 4;
        m -= m / 4;
    }

    if (n >= 1 && m >= 2) {
        unsigned long long int l = n;
        while (l-- > 0) {
            if (l*n <= m) {
                r += l;
                n -= l;
                m -= l*2;
                break;
            }
        }
    }

    printf("%lld\n", r);

    return 0;
}