strange behavior of unsigned long long int in loop

While was solving thisproblem on hackerrank. I noticed a strange thing in for loop. First, let me show you simple code:

#include <bits/stdc++.h>
using namespace std;
#define modVal 1000000007;

int main() {
    for(long long int i=2;i>=0;--i){
        cout<<"here: "<<i<<endl;
    }
}

input: 123

output: here: 2 here: 1 here: 0 164

Now, when I change long long int to unsigned long long int in for loop for the initialization of variable i. The variable i gets initialized with 18446744073709551615. Why is this happening?