#include <stdio.h>
#define nmax 100000

int n,k,m;
int x[nmax],y[nmax];
int i;

int main(){
 freopen("stergeri.in","r",stdin);
 freopen("stergeri.out","w",stdout);
 
 scanf("%d %d %d",&n,&k,&m);
 
 for(i=0;i<m;++i)
    scanf("%d %d",&x[i],&y[i]);
 
 for(i=m-1;i>=0;--i)
    if(x[i] <= k)
        k += y[i] - x[i] + 1;
 
 printf("%d", k);
 
 return 0;
}