tarjan

http://kmplayer.iteye.com/blog/604532

var
        next,last,z,tov,low,dfn:array[1..1000000] of longint;
        vis,zz:array[1..1000000] of boolean;
        tot,n,m,i,x,y,time,zd,ans:longint;
procedure insert(x,y:longint);
begin
        inc(tot);
        tov[tot]:=y;
        next[tot]:=last[x];
        last[x]:=tot;
end;
function min(x,y:longint):longint;
begin
        if x<y then exit(x);
        exit(y);
end;
procedure tarjan(x:longint);
var
        i,y,t:longint;
begin
        inc(time);
        dfn[x]:=time;
        low[x]:=time;
        inc(zd);
        z[zd]:=x;
        zz[x]:=true;
        i:=last[x];
        while i<>0 do
        begin
                y:=tov[i];
                if vis[y] then
                begin
                        vis[y]:=false;
                        tarjan(y);
                        low[x]:=min(low[x],low[y]);
                end
                else
                if zz[y] then low[x]:=min(low[x],dfn[y]);
                i:=next[i];
        end;
        if dfn[x]=low[x] then
        begin
                t:=0;
                repeat
                        zz[z[zd]]:=false;
                        inc(t);
                        dec(zd);
                until z[zd+1]=x;
                inc(ans,t*(t-1)div 2);
        end;
end;
begin
        readln(n,m);
        for i:=1 to m do
        begin
                readln(x,y);
                insert(x,y);
        end;
        fillchar(vis,sizeof(vis),true);
        for i:=1 to n do
                if vis[i] then
                begin
                        vis[i]:=false;
                        tarjan(i);
                end;
                writeln(ans);
end.