Contest1334/I.cpp

#include <bits/stdc++.h>
using namespace std;
#define ONLINE_JUDGE
const int inf = 0x7fffffff;
struct Edge {
  int from, to, nxt;
} e[505];
int idx, n, a, b, d[205], head[205];
void addedge(int from, int to) {
  e[idx].from = from;
  e[idx].to = to;
  e[idx].nxt = head[from];
  head[from] = idx++;
}
void Bellman_Ford() {
  int i, j;
  for (i = 1; i <= n; ++i) {
    d[i] = inf;
  }
  d[a] = 0;
  for (i = 1; i <= n - 1; ++i) {
    for (j = 0; j < idx; ++j) {
      if (d[e[j].from] != inf) {
        if (d[e[j].to] > d[e[j].from] + 1) {
          d[e[j].to] = d[e[j].from] + 1;
        }
      }
    }
  }
  if (d[b] == inf)
    printf("-1\n");
  else
    printf("%d\n", d[b]);
}
int main() {
#ifndef ONLINE_JUDGE
  freopen("G.in", "r", stdin);
  freopen("G.out", "w", stdout);
#endif
  int i, tmp;
  while (scanf("%d", &n) != EOF) {
    if (!n)
      break;
    scanf("%d%d", &a, &b);
    idx = 0;
    memset(head, -1, sizeof(head));
    for (i = 1; i <= n; ++i) {
      scanf("%d", &tmp);
      if (i - tmp > 0)
        addedge(i, i - tmp);
      if (i + tmp <= n)
        addedge(i, i + tmp);
    }
    Bellman_Ford();
  }
  return 0;
}