#include<iostream>
using namespace std;
int main() {
	int N;
	int X[100000];
	int min[2] = { 1000001,0 };
	cin >> N;
	for (int i = 0; i < N; i++) cin >> X[i];
	for (int i = 1; i < N; i++) {
		if (X[i - 1] != X[i])min[1] = X[i] - X[i - 1];
		if (min[1] < 0) min[1] *= -1;
		if (min[1] < min[0])
			min[0] = min[1];
	}
	if (min[0] > 1000000) min[0] = -1;
	cout << min[0] << endl;
}