C++ Program:

Example:

#include <iostream>
using namespace std;

int main() {
  int n;
  cout<<"Enter last number : ";
  cin>>n;
  cout<<"Sum of 1 to "<<n<<" : "<<(n*(n+1))/2;
  return 0;
}

Output:

  Enter last number : 10
  Sum of 1 to 10 : 55

We are using the formula of sum of natural numbers (1 to N)

(N*(N+1))/2  // N is the last number