In C, to calculate the sum of the first N natural numbers (1+2+3+...+N), you can use a loop to iterate through the numbers and add them together.

Here is an example of how this can be done using a for loop:

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#include <stdio.h> int main() {    int n, sum = 0;    printf("Enter the value of n: ");    scanf("%d", &n);    for (int i = 1; i <= n; i++) {        sum += i;    }    printf("Sum of first %d natural numbers: %d", n, sum);    return 0; }

This program prompts the user to enter an integer value for n, and then uses a for loop to iterate through the numbers from 1 to n, adding each number to the sum. Finally, it prints the result.

Alternatively, you can also use the formula to calculate the sum of natural numbers which is N*(N+1)/2,

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#include <stdio.h> int main() {    int n, sum = 0;    printf("Enter the value of n: ");    scanf("%d", &n);    sum = n*(n+1)/2;    printf("Sum of first %d natural numbers: %d", n, sum);    return 0; }

This program also prompts the user to enter an integer value for n and then uses the formula to calculate the sum of the first n natural numbers. Finally, it prints the result.

It's important to note that the first program will have a time complexity of O(n) while the second program will have a time complexity of O(1) which is more efficient and faster.