#include<stdio.h>

long D[50001],L[1000001],i,j,m,n,s,max;

int main (){
    FILE * f = fopen("orase.in","r");
    FILE * g = fopen("orase.out","w");
    fscanf(f,"%ld",&m);
    fscanf(f,"%ld",&n);
    for(i=1;i<=n;i++){
                      fscanf(f,"%ld",&D[i]);
                      fscanf(f,"%ld",&L[i]);
                      }
    max=0;                  
    for(i=1;i<n;i++)
                    for(j=i+1;j<=n;j++){
                                        if(D[i]>D[j])
                                                     s=L[i]+(m-D[j])+L[j];
                                        if(D[i]<D[j])
                                                     s=L[i]+(m-D[i])+L[j];
                                        if(D[i]==D[j])
                                                     s=L[i]+L[j];  
                                        if(s>max)
                                                 max=s;                              
                                        } 
    fprintf(g,"%ld",max);   
    
    
    fclose(f);
    fclose(g);
    
    return 0;
}