Prefix Sum

#include "bits/stdc++.h"

using namespace std;

int main () {
	 ios::sync_with_stdio(false);
	 cin.tie(nullptr);
	 cout.tie(nullptr);
	 int n = 5;
	 int a[n + 1];
	 for (int i = 1; i <= n; ++i) {
		 cin >> a[i];
	 }
	 int p[n + 1];
	 p[0] = 0;
	 int sum = 0;
	 for (int i = 1; i <= n; ++i) {
		 sum += a[i];
		 p[i] = sum;
	 }
	 int L = 2, R = 4;
	 cout << p[R] - p[L - 1] << '\n';
}

//1 2 3 4 5
//1 2 3 4 5 = (L = 2, R = 4) => 9