#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 dy[] = {1, 0, -1, 0}, dx[] = {0, -1, 0, 1};
constexpr int dy8[] = {1, 1, 0, -1, -1, -1, 0, 1}, 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() {
  ll a, b, n, m; cin >> a >> b >> n >> m;
  if (a > b) {
    ll lb = 0, ub = (a - b + n - 1) / n + 1;
    while (ub - lb > 1) {
      ll mid = (lb + ub) / 2;
      (a - b - mid * n >= mid ? lb : ub) = mid;
    }
    cout << b + lb << '\n';
  } else {
    ll lb = 0, ub = (b - a + m - 1) / m + 1;
    while (ub - lb > 1) {
      ll mid = (lb + ub) / 2;
      (b - a - mid * m >= mid ? lb : ub) = mid;
    }
    cout << a + lb << '\n';
  }
  return 0;
}