#include <cstdio>

using namespace std;
short v[4100][4100];
int main()
{
    freopen("triplete.in","r",stdin);
    freopen("triplete.out","w",stdout);
    int n,m,i,j,a,b,sol=0,ok;
    scanf("%d%d",&n,&m);
    for(i=0;i<m;i++)
    {
        scanf("%d%d",&a,&b);
        v[a][b]=1;
    }
    for(i=1;i<=n-2;i++)
    {   ok=0;
        if(v[i][i+1]==1) ok++;
        if(v[i+1][i+2]==1) ok++;
        if(ok==2) sol++;
    }
    printf("%d",sol);
}