Segmentation Fault at a[0]=0

Question

please have a look at my code. My code has a very unfortunate error at

a[0]=0;

https://onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&page=show_problem&problem=2302

    #include <iostream>
    #include <bits/stdc++.h>
    #include <vector>
    #include <string>
    using namespace std;

    #define ll long long int
    #define mod 1e9+7

    ll msum(ll a,ll b,ll m) { return (a%m+b%m)%m; }
    ll msub(ll a,ll b,ll m) { return (a%m-b%m)%m; }
    ll mpro(ll a,ll b,ll m) { return ((a%m)*(b%m))%m; }
    ll fxp(ll a,ll b,ll m) {
        if(b==0)
            return 1;
        if(b%2==0)
            return fxp(a*a,b/2,m);
        return fxp(a,b-1,m)*a;
    }
    void swap(ll &a,ll &b){ ll t=a; a=b; b=t;}

    int main()
    {
        ll t;
        ll n=12158598919;
        ll a[n+1];
        a[0]=0;
        a[1]=1;
        for(ll i=2;i<=n;i++)
            a[n]=i-1;
        for(ll i=2;i<=n;i++)
            for(ll j=2*i;j<=n;j+=i)
                a[j]-=a[i];
        cin>>t;
        while(t!=0)
        {

            ll i,sum=0,k;
            for( i=0;i<=n;i++)
            {
                sum+=a[i];
                if(sum>=t)
                    break;
            }
            t=t-(sum-a[i]);
            ll x=1;
            if(t==0)
                cout<<"0/1"<<endl;
            else
            for(ll j=1;j<i;i++)
            {
                if(__gcd(j,i)==1)
                {
                    x++;
                    if(x==t)
                        cout<<j<<"/"<<i<<endl;
                }
            }
        cin>>t;        
        }
     return 0;
    }