Why does the following code output 1000000000000000000 and not 1000000000000000001?

#include <bits/stdc++.h>
using namespace std;
 
long long inf = 1e18+1;

int main() {
    cout << inf << '\n';
}

I don't understand why there are xt number of colored pixels between y=x and y-axis in Analysis.

Can someone explain Analysis in more detail?

Or can you tell me another solution?