C++ :

#include<iostream>
using namespace std;
int t[301];
int main()
{
	int n;
	cin>>n;
	for (int i=1; i<=n; i++) cin>>t[i];
	int a,b,sum=0;
	cin>>a>>b;
	for (int i=a; i<=b; i++) sum+=t[i];
	cout<<sum<<endl;
	return 0;
}

Pascal :

var n,a,b,i,h:longint;
    l:array[1..300] of longint;
begin
  readln(n);
  for i:=1 to n do
    read(l[i]);
  read(a,b);
  for i:=a to b do
    h:=h+l[i];
  writeln(h);
end.