#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("Yes");}else{puts("No");}
#define MAX_N 200005

int main() {
    int f[4] = {};
    rep(i,2){
        int a;
        cin >> a;
        if(a <= 3)f[a] = 1;
    }
    int cnt = 0;
    rep(i,4)cnt += f[i];
    int ans = -1;
    if(cnt == 2){
        srep(i,1,4){
            if(f[i] == 0)ans = i;
        }
    }
    cout << ans << endl;
    return 0;
}