hdu 5323 Solve this interesting problem 【DFS】

[code]#include <stdio.h>
#include <ctime>
#include <math.h>
#include <limits.h>
#include <complex>
#include <string>
#include <functional>
#include <iterator>
#include <algorithm>
#include <vector>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <list>
#include <bitset>
#include <sstream>
#include <iomanip>
#include <fstream>
#include <iostream>
#include <ctime>
#include <cmath>
#include <cstring>
#include <cstdio>
#include <time.h>
#include <ctype.h>
#include <string.h>
#include <string>
#include <assert.h>

//#pragma comment(linker,"/STACK:1024000000,1024000000")  

using namespace std;

long long l, r;
long long ans;

void dfs(long long x, long long y)
{
    if (x == 0)
    {
        ans = min(ans, y);
        return;
    }
    if (x < 0 || y >= ans)
        return;
    if (x > y) return;

    long long len = y - x + 1;
    if (len > x) return;// 

    if (x != y) dfs(x, y + y - x);
    dfs(x, y + y - x + 1);
    dfs(x + x - y - 1, y);
    dfs(x + x - y - 2, y);
}

int main()
{
    while (~scanf("%lld%lld", &l, &r))
    {
        ans = 1e17;
        if (l > r) printf("-1\n");
        else if (l == r) printf("%d\n", l);
        else
        {
            dfs(l, r);
            if (ans == 1e17) ans = -1;
            printf("%lld\n",ans);
        }
    }
    return 0;
}