#include <bits/stdc++.h>

struct FastIO {
	char buf[1000000];
	char *ptr = buf;
	void read() {
		int size;
		size = fread_unlocked(buf, 1, sizeof(buf) - 1, stdin);
		buf[size] = '\0';
		ptr = buf;
	}
	FastIO () {
		read();
	}
	void inc() {
		if (++ptr == buf + sizeof(buf) || !*ptr) read();
	}
	template<typename Integer> Integer read() {
		bool neg = false;
		Integer res = 0;
		while ((*ptr < '0' || *ptr > '9') && *ptr != '-') inc();
		if (*ptr == '-') neg = true, inc();
		while (*ptr >= '0' && *ptr <= '9') res = res * 10 + *ptr - '0', inc();
		return neg ? -res : res;
	}
} fastio;
#define ri fastio.read<int>
#define rs64 fastio.read<int64_t>

int main() {
	int x0 = ri(), y0 = ri(), x1 = ri(), y1 = ri();
	printf("%.7f\n", y0 + (double) (y1 - y0) * x0 / (x0 + x1));
	return 0;
}