#include <cstdio>
#include <algorithm>
struct Edge {int from, to;Edge* next;
};
const int MAXN = 100001, MAXM = 500001;
struct UFS {int father[MAXN], rank[MAXN], root(int);void initialize(int), plus(int, int);
};
int n, m, low[MAXN], dfn[MAXN], nowdfn, stk[MAXN], top, s[MAXN];
Edge* head[MAXN];
bool vis[MAXN], ins[MAXN];
Edge edges[MAXM];
UFS ufs;
void dfs(int);
int main() {int cnt = 0, tmp;scanf("%d%d", &n, &m);for(int i = 1; i <= m; i++) {int s, t;Edge* tmp = new Edge;scanf("%d%d", &s, &t);tmp->from = s;tmp->to = t;tmp->next = head[s];head[s] = tmp;}ufs.initialize(n);for(int i = 1; i <= n; i++) {if(!vis[i]) {dfs(i);}}for(int i = 1; i <= n; i++) {for(Edge* j = head[i]; j != NULL; j = j->next) {if(ufs.root(i) != ufs.root(j->to)) {s[ufs.root(i)]++;}}}for(int i = 1; i <= n; i++) {if(ufs.root(i) == i && s[i] == 0) {cnt++;tmp = i;}}printf("%d\n", cnt == 1 ? ufs.rank[tmp] : 0);return 0;
}
void UFS::initialize(int n) {for(int i = 1; i <= n; i++) {father[i] = i;rank[i] = 1;}
}
int UFS::root(int x) {return father[x] == x ? x : root(father[x]);
}
void UFS::plus(int x, int y) {father[y] = x;rank[x] += rank[y];
}
void dfs(int x) {nowdfn++;top++;dfn[x] = low[x] = nowdfn;ins[x] = vis[x] = true;stk[top] = x;for(Edge* i = head[x]; i != NULL; i = i->next) {int go = i->to;if(!vis[go]) {dfs(go);low[x] = std::min(low[x], low[go]);}if(ins[go]) {low[x] = std::min(low[x], dfn[go]);}}if(dfn[x] == low[x]) {int f = stk[top];top--;if(f != x) {while(true) {int i = stk[top];top--;ufs.plus(f, i);if(i == x) {break;}}}}
}

#include <cstdio>
#include <algorithm>
struct Edge {int from, to;Edge* next;
};
const int MAXN = 100001, MAXM = 500001;
struct UFS {int father[MAXN], rank[MAXN], root(int);void initialize(int), plus(int, int);~UFS();
};
struct Cell {int x;bool finished, first, visjto;Edge* j;void call();
};
int n, m, low[MAXN], dfn[MAXN], nowdfn, stk[MAXN], top, s[MAXN], cst;
bool vis[MAXN], ins[MAXN];
Edge* head[MAXN];
UFS ufs;
Cell cs[MAXN];
int main() {int cnt = 0, tmp;freopen("popular.in", "r", stdin);freopen("popular.out", "w", stdout);scanf("%d%d", &n, &m);for(int i = 1; i <= m; i++) {int s, t;Edge* tmp = new Edge;scanf("%d%d", &s, &t);tmp->from = s;tmp->to = t;tmp->next = head[s];head[s] = tmp;}ufs.initialize(n);for(int i = 1; i <= n; i++) {if(!vis[i]) {cst = 1;cs[cst].x = i;cs[cst].first = true;cs[cst].finished = false;while(cst != 0) {cs[cst].call();if(cs[cst].finished) {cst--;}}}}for(int i = 1; i <= n; i++) {for(Edge* j = head[i]; j != NULL; j = j->next) {if(ufs.root(i) != ufs.root(j->to)) {s[ufs.root(i)]++;}}}for(int i = 1; i <= n; i++) {if(ufs.root(i) == i && s[i] == 0) {cnt++;tmp = i;}}printf("%d\n", cnt == 1 ? ufs.rank[tmp] : 0);for(int i = 1; i <= n; i++) {while(head[i] != NULL) {Edge* tmp = head[i]->next;delete head[i];head[i] = tmp;}}fclose(stdin);fclose(stdout);return 0;
}
void UFS::initialize(int n) {for(int i = 1; i <= n; i++) {father[i] = i;rank[i] = 1;}
}
int UFS::root(int x) {return father[x] == x ? x : root(father[x]);
}
void UFS::plus(int x, int y) {father[y] = x;rank[x] += rank[y];
}
UFS::~UFS() {delete[] father;delete[] rank;
}
void Cell::call() {if(first) {nowdfn++;top++;dfn[x] = low[x] = nowdfn;ins[x] = vis[x] = true;stk[top] = x;j = head[x];first = false;}else {if(!visjto) {low[x] = std::min(low[x], low[j->to]);}if(ins[j->to]) {low[x] = std::min(low[x], dfn[j->to]);}j = j->next;}if(j == NULL) {if(low[x] == dfn[x]) {int f = stk[top];top--;if(f != x) {while(true) {int i = stk[top];top--;ufs.plus(f, i);if(i == x) {break;}}}}finished = true;}else {if(!vis[j->to]) {cst++;cs[cst].x = j->to;cs[cst].first = true;cs[cst].finished = false;visjto = false;}else {visjto = true;}}
}