#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
int main(){
    int N;
    vector<int> X(N);
    cin >> N;
    for(int i=0;i<N;i++)cin >> X[i];
    sort(X.begin(),X.end());
    
    int ans=1e6+1;
    if(X[0]==X[N-1])ans=-1;
    else for(int k=0;k+1<N;k++){
        if(X[k]!=X[k+1])ans=min(ans,X[k+1]-X[k]);
    }
    cout << ans << endl;
    return 0;
}