#include <bits/stdc++.h>   bool vis[100000 + 3];   int pr[20000];   int h;   int a[300];   int n;   int endp[200000], next[200000], cap[200000];   int gra[300], cur[300], dist[300], prev[300];   int cnt[300];   int ecnt;   char buf[10000], *bend;   char out[10000], *oend;   void add_edge(int u, int v, int c) {     endp[++ecnt] = v;     cap[ecnt] = c;     next[ecnt] = gra[u];     gra[u] = ecnt;     endp[++ecnt] = u;     cap[ecnt] = 0;     next[ecnt] = gra[v];     gra[v] = ecnt;   }   int isap(int vcnt, int S_, int T_) {     memmove(cur, gra, sizeof(gra));     memset(cnt, 0, sizeof(cnt));     memset(dist, 0, sizeof(dist));     prev[S_] = S_;     int ret = 0, top;     int u = S_;     while (dist[S_] < vcnt) {       int i = cur[u];       for (; ~i; i = next[i]) {         int v = endp[i];         if (cap[i] && dist[u] == dist[v] + 1) {           prev[v] = cur[u] = i;           u = v;           break;         }       }       if (!~i) {         cur[u] = gra[u];         top = vcnt;         for (int i = gra[u]; ~i; i = next[i]) {           int v = endp[i];           if (cap[i] && dist[v] < top) {             top = dist[v];           }         }         ++cnt[top + 1];         if (!--cnt[dist[u]]) {           goto STOP;         }         dist[u] = top + 1;         if (u != S_) {           u = endp[prev[u] ^ 1];         }       }       if (u == T_) {         int arg = 1 << 29;         while (u != S_) {           if (cap[prev[u]] < arg) {             arg = cap[prev[u]];           }           u = endp[prev[u] ^ 1];         }         ret += arg;         while (u != T_) {           cap[cur[u]] -= arg;           cap[cur[u] ^ 1] += arg;           u = endp[cur[u]];         }         u = S_;       }     }   STOP:     return ret;   }   int find(int u, int p) {     vis[u] = true;     sprintf(bend,   %d , u + 1);     bend += strlen(bend);     *bend = 0;     for (int i = gra[u]; ~i; i = next[i]) {       int v = endp[i];       if (v == p || vis[v]) {         continue;       }       if ((!!cap[i]) ^ (a[u] & 1)) {         return find(v, u) + 1;       }     }     return 1;   }   bool is_pr(int val) {     int l = 0, r = h - 1;     while (r - l > 1) {       int mid = (l + r) >> 1;       if (pr[mid] < val) {         l = mid;       } else {         r = mid;       }     }     return pr[r] == val;   }   int main() {     for (int i = 2; i < 100000; ++i) {       if (!vis[i]) {         pr[h++] = i;       }       for (int j = 0; j < h; ++j) {         if (100000 / pr[j] < i) {           break;         }         vis[pr[j] * i] = true;         if (!(i % pr[j])) {           break;         }       }     }     while (scanf( %d , &n) != EOF) {       for (int i = 0; i < n; ++i) {         scanf( %d , a + i);       }       memset(gra, 255, sizeof(gra));       ecnt = -1;       for (int i = 0; i < n; ++i) {         if (a[i] & 1) {           add_edge(n, i, 2);           for (int j = 0; j < n; ++j) {             if (!(a[j] & 1) && is_pr(a[i] + a[j])) {               add_edge(i, j, 1);             }           }         } else {           add_edge(i, n + 1, 2);         }       }       int flow = isap(n + 2, n, n + 1);       if (flow != n) {         puts( Impossible );       } else {         oend = out;         memset(vis, 0, sizeof(vis));         int cnt = 0;         for (int i = 0; i < n; ++i) {           if (!vis[i]) {             bend = buf;             sprintf(oend,  %d%s n , find(i, -1), buf);             oend += strlen(oend);             ++cnt;           }         }         *(++oend) = 0;         printf( %d n%s , cnt, out);       }     }     return 0;   }