// ここから「ここまでテンプレ」までテンプレ
#define NOMINMAX
#define TEST_MODE true

#define _CRT_SECURE_NO_WARNINGS
#define _USE_MATH_DEFINES

#include "bits/stdc++.h"
#include <random>
#include <regex>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); ++i)
#define rep2(i, a, b) for (int i = (a); i < (int)(b); ++i)
#define rrep(i, n) for (int i = (n)-1; i >= 0; --i)
#define rrep2(i, a, b) for (int i = (a)-1; i >= (int)b; --i)
#define range(i, a, b, c) for (int i = a;             \
							   c > 0 ? i < b : i > b; \
							   i += c)
#define chmax(a, b) (a = (a) < (b) ? (b) : (a))
#define chmin(a, b) (a = (a) > (b) ? (b) : (a))
using ll = long long;
using ull = unsigned long long;
using ld = long double;
#define all(a) begin(a), end(a)
#define ifnot(a) if (not(a))
#define int long long

#ifdef LOCAL_ENV

#if TEST_MODE == true
const bool test = true;
#define dump(x) cerr << #x << " : " << (x) << " "
#define dumpl(x) dump(x) << endl
#else
const bool test = false;
#define dump(x)
#define dumpl(x)
#endif

#else
const bool test = false;
#define dump(x)
#define dumpl(x)
#endif

int dx[] = { 1, 0, -1, 0 };
int dy[] = { 0, 1, 0, -1 };
const int inf = (int)1 << 60;
const int undefined = inf;
const ll INFL = (ll)1 << 60;
ll mod_n = (int)1e9 + 7;
typedef long double Real;
const Real eps = 1e-10;
// return -1, 0, 1
int sgn(const Real& r) { return (r > eps) - (r < -eps); }
int sgn(const Real& a, const Real& b) { return sgn(a - b); }

//.....................
const int MAX = (int)2e5 + 5;
const int MAX2 = (int)3e2;

vector<string> split(const string& str, char sep)
{
	vector<string> v;
	stringstream ss(str);
	string buffer;
	while (getline(ss, buffer, sep))
	{
		v.push_back(buffer);
	}
	return v;
}

template <typename T>
string join(const vector<T>& v, const string delim = 0, function<string(T)> f = [](int a) { return to_string(a); })
{
	string s;
	if (!v.empty())
	{
		s += f(v[0]);
		for (decltype(v.size()) i = 1, c = v.size(); i < c; ++i)
		{
			if (delim != "")
				s += delim;
			s += f(v[i]);
		}
	}
	return s;
}

string join_str(const vector<string>& v, const string delim = 0)
{
	return join<string>(v, delim, [](string a) { return a; });
}

string operator*(const string& s, const int& n)
{
	string res = "";
	rep(i, n) res += s;
	return res;
}

template <class InputIterator>
int sum(InputIterator begin, InputIterator end)
{
	return accumulate(begin, end, 0ll);
}

template <typename T>
T gcd(T a, T b)
{
	if (a == inf)
		return b;
	if (b == inf)
		return a;
	T c;
	while (a != 0)
	{
		c = a;
		a = b % a;
		b = c;
	}
	return b;
}

template <typename T>
T lcm(T m, T n)
{
	return ((m / gcd(m, n)) * n);
}

template <typename T>
T bit_count(T N)
{
	T cnt = 0;
	while (N)
	{
		if (N & 1)
			cnt++;
		N >>= 1;
	}
	return cnt;
}

template <typename T>
void print_vec(ostream& ostream, vector<T> a)
{
	rep(i, a.size() - 1) ostream << a[i] << " ";
	ostream << a.back() << endl;
}

template <typename T>
void print_enumerable(ostream& ostream, T a_)
{
	for (auto a : a_)
	{
		cerr << a << " ";
	}
	cerr << endl;
}

ll pow_n(ll x, ll n)
{
	ll res = 1;
	while (n > 0)
	{
		if (n & 1)
			res = res * x;
		x = x * x;
		n >>= 1;
	}
	return res;
}

template <class T, class U>
T mod_pow(T x, U n)
{
	T res = 1;
	while (n > 0)
	{
		if (n & 1)
			res = res * x;
		x = x * x;
		x %= mod_n;
		n >>= 1;
		res %= mod_n;
	}
	return res;
}

template <class T>
T mod_rev(T a)
{
	T res = 1;
	return mod_pow(a, mod_n - 2);
}

int H, W;

bool grid_ng(int y, int x)
{
	return y < 0 || y >= H || x < 0 || x >= W;
}

struct Point
{
	int x, y;
	Point(int y_, int x_) : y(y_), x(x_) {}
	bool operator<(const Point& r) const
	{
		if (y != r.y)
			return y < r.y;
		return x < r.x;
	}
};

struct Vertex
{
	int id, dist;
	bool operator>(const Vertex& r) const
	{
		return dist > r.dist;
	}
};

struct Edge
{
	int to, weight;
	Edge(int to, int weight) : to(to), weight(weight) {}
};

using Graph = vector<vector<Edge>>;
Graph g;

// ここまでテンプレ

int n, k;
string s;
int l;

class Solver
{
public:

	void solve() {
		int a, b; cin >> a >> b;
		rep(i, 100001) {
			if (i * 8 / 100 == a
				&& i * 10 / 100 == b) {
				cout << i << endl;
				return;
			}
		}
		cout << -1 << endl;
	}
};

// ここから最後までテンプレ

signed main()
{
	srand((unsigned int)time(NULL));
	cout << fixed << setprecision(20);
	cerr << fixed << setprecision(3);
	auto ptr = new Solver();
	ptr->solve();
	delete ptr;

	//while(true) {
	//	auto ptr = new Solver();
	//	ptr->solve();
	//	delete ptr;
	//}
	//while (true) {
	//	if (cin.eof() == true) break;
	//	auto ptr = new Solver();
	//	ptr->solve();
	//	delete ptr;
	//}
	return 0;
}


Measured memory_bytes: 256