大水题，暴力加即可。

#include <bits/stdc++.h>
using namespace std;

inline int read() {
	int x = 0, f = 0; char c = 0;
	while (!isdigit(c)) f |= c == '-', c = getchar();
	while (isdigit(c)) x = (x << 3) + (x << 1) + (c ^ 48), c = getchar();
	return f ? -x : x;
}

#define N 110

int n, a, b, res = 0, d[N];

int main() {
    n = read();
    for (int i = 1; i < n; i ++) {
        d[i] = read();
    }
    a = read(), b = read();
    for (int i = a; i < b; i ++) {
        res += d[i];
    }
    printf("%d\n", res);
    return 0;
}

我就是来水一篇题解的