#include <bits/stdc++.h>
using namespace std;
#define _ <<" "<<
#define sz(x) ((int)x.size())
typedef long long ll;
int n;
typedef pair<int,int> pii;

namespace IO {
#define gc getchar()
#define pc(x) putchar(x)
    template<typename T>inline void read(T &x) {
        x=0;
        int f=1;
        char ch=gc;
        while(ch>'9'||ch<'0') {
            if(ch=='-')f=-1;
            ch=gc;
        }
        while(ch>='0'&&ch<='9')x=(x<<3)+(x<<1)+ch-'0',ch=gc;//如果读入就要取模，就加一句x=x%mod
        x*=f;
        return;
    }
    template<typename T>inline void write(T x=0) {
        T wr[51];
        wr[0]=0;
        if(x<0)pc('-'),x=-x;
        if(!x)pc(48);
        while(x)wr[++wr[0]]=x%10,x/=10;
        while(wr[0])pc(48+wr[wr[0]--]);
        return;
    }
}
using IO::read;
using IO::write;


#define debug(x)  cout<<#x<<':'<<x<<endl;

const int maxn=1e5+100;
int c[maxn];
const int M=1<<25;


int main(){
   
    int t,s,n;
    read(t);
    read(s);
    read(n);

    vector<ll>a(n+1);
    ll up=0;ll down=s;
    a[n]=t;
    int last=0;
    for(int i=0;i<n+1;i++){
        
        cin>>a[i];

        ll delta=a[i]-last;

        ll imin=min(up,delta);
        up-=imin;down+=imin;
        swap(up,down);
        last=a[i];
       // debug(up);debug(down);
    }


    cout<<down<<endl;
    

    

    
}