#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
using ll = long long;
constexpr int INF = 0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
constexpr double EPS = 1e-8;
constexpr int MOD = 1000000007;
// constexpr int MOD = 998244353;
constexpr int DY4[]{1, 0, -1, 0}, DX4[]{0, -1, 0, 1};
constexpr int DY8[]{1, 1, 0, -1, -1, -1, 0, 1};
constexpr int DX8[]{0, -1, -1, -1, 0, 1, 1, 1};
template <typename T, typename U>
inline bool chmax(T& a, U b) { return a < b ? (a = b, true) : false; }
template <typename T, typename U>
inline bool chmin(T& a, U b) { return a > b ? (a = b, true) : false; }
struct IOSetup {
  IOSetup() {
    std::cin.tie(nullptr);
    std::ios_base::sync_with_stdio(false);
    std::cout << fixed << setprecision(20);
  }
} iosetup;

int main() {
  constexpr int N = 4, F = 4;
  int fruits[N - 1]{}; REP(i, N - 1) cin >> fruits[i];
  REP(d, F) {
    int tmp[N];
    copy(fruits, fruits + (N - 1), tmp);
    tmp[N - 1] = d;
    sort(tmp, tmp + N);
    if (tmp[0] == tmp[N - 1] || (tmp[0] == tmp[1] && tmp[2] == tmp[3]) || (tmp[0] < tmp[1] && tmp[1] < tmp[2] && tmp[2] < tmp[3])) {
      cout << d << '\n';
      return 0;
    }
  }
  assert(false);
}