#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
template <class S> using v = vector<S>;
template <class S> using vv = vector<vector<S>>;
template <class S> using vvv = vector<vector<vector<S>>>;
#define rep(...) overload_rep(__VA_ARGS__, rep3, rep2)(__VA_ARGS__)
#define overload_rep(_1, _2, _3, name, ...) name
#define rep2(i, n) for (int i = 0; i < (int)(n); i++)
#define rep3(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
#define rrep(i,n) for(int i=(int)(n)-1;i>=0;i--)
#define all(x) (x).begin(), (x).end()
#define vi vector<int>
#define vll vector<ll>
#define vvi vector<vector<int>>
#define vvll vector<vector<ll>>
#define vvvi vector<vector<vector<int>>>
#define pii pair<int, int>
#define pll pair<ll, ll>
#define el '\n'
#define sp ' '
#define Yes cout<<"Yes"<<endl
#define YES cout<<"YES"<<endl
#define No cout<<"No"<<endl
#define NO cout<<"NO"<<endl
template<class T>
bool chmin(T &a, const T &b){
    if (b < a){ a = b; return true; }
    return false;
}
template<class T>
bool chmax(T &a, const T &b){
    if (b > a){ a = b; return true; }
    return false;
}

using mtup = tuple<int, int, int>;
const int INF = 1e9;
const long long LINF = 1e18;
const int mod = 998244353;
// const int mod = 1e9 + 7;

ll extgcd(ll a, ll b, ll &x, ll &y) {
    if (b == 0) {
        x = 1;
        y = 0;
        return a;
    }
    ll x1, y1;
    ll g = extgcd(b, a % b, x1, y1);
    x = y1;
    y = x1 - (a / b) * y1;
    return g;
}

ll mod_inv(ll a, ll mod) {
    ll x, y;
    extgcd(a, mod, x, y);
    x %= mod;
    if (x < 0) x += mod;
    return x;
}

ll CRT(vi& a, vi&m){
    ll M = 1, res = 0; 
    for(auto i: m) M *= i;
    for(int i = 0; i < a.size(); ++i){
        ll Mi = M / m[i];
        ll xi = mod_inv(Mi, m[i]);
        ll ei = Mi * xi % M;
        res = (res + a[i] * ei % M) % M;
    }
    return res;
}

vi f(vi a){
    int n = 2;
    int m = a.size();
    vi res = a;
    rep(i, n - 1){
        rep(j, m){
            res[j] = a[res[j] - 1];
        }
        a = res;
    }
    return res;
}

int main(){
    //高速化
    //ここから

    vi a(2), mod(2); 
    cin >> mod[0] >> mod[1];
    cin >> a[0] >> a[1];
    cout << CRT(a, mod) << el;
}
/*
cppt
cppt_ge
seg, bit, uf

g++ main.cpp -O2 -std=c++17
./a.out

*/