1 #include<iostream>  2 #include<cstdio>  3 #include<cstring>  4 #include<algorithm>  5 #include<queue>  6 using namespace std;  7 const int N=205,INF=0x3f3f3f3f;  8 int bmap[N][N],cx[N],cy[N],dx[N],dy[N];  9 bool bmask[N]; 10 int flag,vis[N]; 11 int nx,ny,dis,ans; 12 bool searchpath() 13 { 14     queue<int> q; 15     dis=INF; 16     memset(dx,-1,sizeof(dx)); 17     memset(dy,-1,sizeof(dy)); 18     for(int i=1;i<=nx;i++) 19     { 20         if(cx[i]==-1){ q.push(i); dx[i]=0; } 21         while(!q.empty()) 22         { 23             int u=q.front(); q.pop(); 24             if(dx[u]>dis) break; 25             for(int v=1;v<=ny;v++) 26             { 27                 if(bmap[u][v]&&dy[v]==-1) 28                 { 29                     dy[v]= dx[u] + 1; 30                     if(cy[v]==-1) dis=dy[v]; 31                     else 32                     { 33                         dx[cy[v]]= dy[v]+1; 34                         q.push(cy[v]); 35                     } 36                 } 37             } 38         } 39     } 40     return dis!=INF; 41 } 42 int findpath(int u) 43 { 44     for(int v=1;v<=ny;v++) 45     { 46         if(!bmask[v]&&bmap[u][v]&&dy[v]==dx[u]+1) 47         { 48             bmask[v]=1; 49             if(cy[v]!=-1&&dy[v]==dis) continue; 50             if(cy[v]==-1||findpath(cy[v])) 51             { 52                 cy[v]=u; cx[u]=v; 53                 return 1; 54             } 55         } 56     } 57     return 0; 58 } 59 void maxmatch() 60 { 61     ans=0; 62     memset(cx,-1,sizeof(cx)); 63     memset(cy,-1,sizeof(cy)); 64     while(searchpath()) 65     { 66         memset(bmask,0,sizeof(bmask)); 67         for(int i=1;i<=nx;i++) 68             if(cx[i]==-1) ans+=findpath(i); 69     } 70 } 71 void init() 72 { 73     memset(bmap,0,sizeof(bmap)); 74 } 75 void dfs(int u) 76 { 77     for(int i=1;i<=nx&&!flag;i++) 78     { 79         if(u==i) continue; 80         if(bmap[u][i]){ 81             if(vis[i]!=-1&&(vis[u]^vis[i]!=1)) flag=1; 82             if(vis[i]==-1) {vis[i]=vis[u] ^1;dfs(i);} 83         } 84     } 85 } 86 int main() 87 { 88     //freopen("test.txt","r",stdin); 89     int cas,i,j,k,m,n; 90     while(scanf("%d%d",&n,&m)!= EOF) 91     { 92         init(); 93         memset(vis,-1,sizeof(vis)); 94         flag=0; 95         for(i=1;i<=m;i++) 96         { 97             scanf("%d%d",&j,&k); 98             bmap[j][k]=1; 99         }100         nx=ny=n;101         vis[1]=0;102         dfs(1);103         if(flag) {printf("No\n");continue;}104         maxmatch();105         printf("%d\n",ans);106     }107     return 0;108 }