#include<bits/stdc++.h>
#define ALL(v) std::begin(v),std::end(v)
using lint=long long;
using lubl=long double;
template<class Begin,class End>
struct span{
    using value_type=typename Begin::value_type;
    Begin begin_;
    End end_;
    span(Begin begin__, End end__):begin_(begin__),end_(end__){}
    auto begin(){return begin_;}
    auto end(){return end_;}
    auto begin()const{return begin_;}
    auto end()const{return end_;}
};
int main(){
    std::cin.tie(nullptr);std::ios_base::sync_with_stdio(false);
    std::cout.setf(std::ios_base::fixed);std::cout.precision(15);
    lint n,m;std::cin>>n>>m;
    lint X=7*24*60;
    auto read=[]{
        char _;lint d,h,min;
        std::cin>>d>>h>>_>>min;
        return ((d-2)*24+h)*60+min;
    };
    std::vector<std::pair<lint,lint>>a(m);
    for(lint i=0;i<m;i++){
        lint l=read(),r=read();
        a.at(i)={r+1,l};
    }
    std::sort(ALL(a));
    std::vector<lint>occ(X);
    lint ans=0;
    for(auto&&p:a){
        lint r,l;std::tie(r,l)=p;
        span sp(occ.begin()+l,occ.begin()+r);
        if(std::any_of(ALL(sp),[n](lint x){return x==n;}))continue;
        std::for_each(ALL(sp),[](lint&x){x++;});
        ans++;
    }
    std::cout<<ans<<'\n';
}