min step to reach destination

#include <iostream>
using namespace std;
int minstep(int a[],int n){
    int i;
    if(n<=1)return 0;
    if(a[0]==0)return -1;
    int step=a[0],jump=1,dis=a[0];
    for(i=1;i<n;i++){
        if(i==n-1)return jump;
        dis=max(dis,a[i]+i);
        step--;
        if(step==0){
            jump++;
            if(dis<=i)return -1;
            step=dis-i;
        }
    }
}
int main() {
    int i,j,t,n;
    cin>>t;
    while(t--){
        cin>>n;
        int a[n];
        for(i=0;i<n;i++)cin>>a[i];
        cout<<minstep(a,n)<<endl;
    }

    return 0;
}