#define _CRT_SECURE_NO_WARNINGS
#pragma comment(linker, "/stack:512000000")
#include<utility>
#include<iostream>
#include<fstream>
#include<vector>
#include<array>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
#include<ctime>
#include<functional>
#include<random>
#include<climits>
#include<queue>
#include<stack>
#include<deque>
#include<bitset>
#include<map>
#include<set>
#include<unordered_map>
#include<unordered_set>
#include<cassert>
#include<ccomplex>
#include<chrono>
#include<iomanip>

#ifdef ONLINE_JUDGE
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,tune=native")
#pragma GCC optimize("unroll-loops")
#define int128 __int128
#else
#define int128 long long
#endif

#define all(x) x.begin(), x.end()
#define make_unique(x) sort(all((x))); (x).resize(unique(all((x))) - (x).begin())
#define mp make_pair
#define X first
#define Y second

using namespace std;

using ll = long long;
using ull = unsigned long long;
using ld = long double;
mt19937_64 gen(time(nullptr));

namespace {
    struct custom_hash {
        static uint64_t splitmix64(uint64_t x) {
            // http://xorshift.di.unimi.it/splitmix64.c
            x += 0x9e3779b97f4a7c15;
            x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
            x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
            return x ^ (x >> 31);
        }

        size_t operator()(uint64_t x) const {
            static const uint64_t FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();
            return splitmix64(x + FIXED_RANDOM);
        }
    };

    template<typename T, typename F>
    using HashMap = unordered_map<T, F, custom_hash>;

    template<typename T>
    using HashSet = unordered_set<T, custom_hash>;
}

ll mod = 1e9 + 7;

namespace {
    ll mul(ll a, ll b) {
        ll val = a * b - (ll)((ld)a * b / mod) * mod;
        if (val < 0) val += mod;
        if (val >= mod) val -= mod;
        return val;
    }
    ll poww(ll a, ll b) {
        ll val = 1;
        a %= mod;
        while (b > 0) {
            if (b % 2) val = mul(a, val);
            a = mul(a, a);
            b >>= 1;
        }
        return val % mod;
    }
    ll inv(ll a) {
        return poww(a, mod - 2);
    }
    ll gcd(ll a, ll b) {
        return (a == 0 ? b : gcd(b % a, a));
    }
    ll gcdex(ll a, ll b, ll& x, ll& y) {
        if (a == 0) {
            x = 0; y = 1;
            return b;
        }
        ll x1, y1;
        ll d = gcdex(b % a, a, x1, y1);
        x = y1 - (b / a) * x1;
        y = x1;
        return d;
    }

    ll inv_eu(ll a, ll m) {
        ll x, y;
        ll g = gcdex(a, m, x, y);
        if (g != 1)
            assert(0);
        else {
            x = (x % m + m) % m;
            return x;
        }
    }
}

ll const inf = 1e11 + 9;
ll const maxk = 22;
ll const maxn = 3e5 + 5;


signed main() {
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);

    ld sx, sy, gx, gy;
    cin >> sx >> sy >> gx >> gy;

    ld len = gx - sx;
    

    cout << fixed << setprecision(15);

    cout << (sx + len / (1 + gy / sy));
    return 0;
}

/*
1
8
1 2 7 3 2 1 2 3


1
7
4 1 5 2 6 3 7

*/
/********   All Required Header Files ********/
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <sstream>
#include <queue>
#include <deque>
#include <bitset>
#include <iterator>
#include <list>
#include <stack>
#include <map>
#include <set>
#include <functional>
#include <numeric>
#include <utility>
#include <limits>
#include <time.h>
#include <math.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <assert.h>

using namespace std;

/*******  All Required define Pre-Processors and typedef Constants *******/
#define SCD(t) scanf("%d",&t)
#define SCLD(t) scanf("%ld",&t)
#define SCLLD(t) scanf("%lld",&t)
#define SCC(t) scanf("%c",&t)
#define SCS(t) scanf("%s",t)
#define SCF(t) scanf("%f",&t)
#define SCLF(t) scanf("%lf",&t)
#define MEM(a, b) memset(a, (b), sizeof(a))
#define FOR(i, j, k, in) for (int i=j ; i<k ; i+=in)
#define RFOR(i, j, k, in) for (int i=j ; i>=k ; i-=in)
#define REP(i, j) FOR(i, 0, j, 1)
#define RREP(i, j) RFOR(i, j, 0, 1)
#define all(cont) cont.begin(), cont.end()
#define rall(cont) cont.end(), cont.begin()
#define FOREACH(it, l) for (auto it = l.begin(); it != l.end(); it++)
#define IN(A, B, C) assert( B <= A && A <= C)
#define MP make_pair
#define PB push_back
#define INF (int)1e9
#define EPS 1e-9
#define PI 3.1415926535897932384626433832795
#define MOD 1000000007
#define read(type) readInt<type>()
const double pi=acos(-1.0);
typedef pair<int, int> PII;
typedef vector<int> VI;
typedef vector<string> VS;
typedef vector<PII> VII;
typedef vector<VI> VVI;
typedef map<int,int> MPII;
typedef set<int> SETI;
typedef multiset<int> MSETI;
typedef long int int32;
typedef unsigned long int uint32;

/****** Template of some basic operations *****/
template<typename T, typename U> inline void amin(T &x, U y) { if(y < x) x = y; }
template<typename T, typename U> inline void amax(T &x, U y) { if(x < y) x = y; }
/**********************************************/

/****** Template of Fast I/O Methods *********/
template <typename T> inline void write(T x)
{
	int i = 20;
	char buf[21];
	// buf[10] = 0;
	buf[20] = '\n';

	do
	{
		buf[--i] = x % 10 + '0';
		x/= 10;
	}while(x);
	do
	{
		putchar(buf[i]);
	} while (buf[i++] != '\n');
}

template <typename T> inline T readInt()
{
	T n=0,s=1;
	char p=getchar();
	if(p=='-')
		s=-1;
	while((p<'0'||p>'9')&&p!=EOF&&p!='-')
		p=getchar();
	if(p=='-')
		s=-1,p=getchar();
	while(p>='0'&&p<='9') {
		n = (n<< 3) + (n<< 1) + (p - '0');
		p=getchar();
	}

	return n*s;
}
/************************************/


/******* Debugging Class Template *******/
#define DEBUG

#ifdef DEBUG

    #define debug(args...)     (Debugger()) , args

    class Debugger
    {
        public:
        Debugger(const std::string& _separator = " - ") :
        first(true), separator(_separator){}

        template<typename ObjectType> Debugger& operator , (const ObjectType& v)
        {
            if(!first)
                std:cerr << separator;
            std::cerr << v;
            first = false;
            return *this;
        }
        ~Debugger() {  std:cerr << endl;}

        private:
        bool first;
        std::string separator;
    };

#else
    #define debug(args...)                  // Just strip off all debug tokens
#endif

#define ll long long
#define MAX_PRIME 100005

vector<int> primes;

void sieve(){
	bool composite[MAX_PRIME];
	composite[0] = true;
	composite[1] = true;
	for(int i=2; i*i<MAX_PRIME; i++){
		if(!composite[i]){
			primes.push_back(i);
			for(int j=2*i; j<MAX_PRIME; j+=i) composite[j]=true;
		}
	}
}

void solve(){
	ll sx, sy, gx, gy;
	cin>>sx>>sy>>gx>>gy;
	double ans=(double)(sy*gx+gy*sx)/(double)(sy+gy);
	cout<<std::fixed;
	cout.precision(9);
	cout<<ans<<"\n";
}

int main()
{
	int t=1;
	// cin>>t;
	while(t--){
		solve();
	}
	return 0;
}