Nothing getting displayed on output

when i am trying to run the code by giving inputs, it is not showing anything
the code is simple to print the array that is given in input-
#include
using namespace std;
int main() {
long long int n;
int a[1000][1000];
int i,j;
for(i=0;i<n;i++)
{
for(j=0;j<n;j++)
{
cin>>a[i][j];
}
}
for(i=0;i<n;i++)
{
for(j=0;j<n;j++)
{
cout<<a[i][j];
}
}
return 0;
}