Sum of n numbers

Input   N (The numbers from 1 to N will be added)        
Output  Sum of first n numbers       
complexity O(1)    
  
Sum_Num(N)  
1 Sum=N*(N+1)/2;  
2 Print sum;  
 

//To find the sum of 1+2+3....+n terms
//Input : Enter any positive integer n

#include< stdio.h>
#include< conio.h>
int main()
{
    int sum,n;
    printf("Enter a number upto which sum is required ");
    scanf("%d",&n);
    sum=n*(n+1)/2;
    printf("Sum from 1 to %d is %d",n,sum);
}