Find sum of natural numbers without formula
# take input from the user
num = as.integer(readline(prompt = "Enter a number: "))

if(num < 0) {
    print("Enter a positive number")
} else {
    sum = 0

    # use while loop to iterate until zero
    while(num > 0) {
        sum = sum + num
        num = num - 1
    }

    print(paste("The sum is", sum))
}
Output

Enter a number: 10
[1] "The sum is 55"