#include "bits/stdc++.h"
using namespace std;
#define intt long long
#define fi first
#define se second
const intt mxN = 2e5 + 5;
const intt LG = 20;
const intt inf = 1e18;
intt x, n;
void _() {
cin >> x >> n;
intt cur = x;
for(intt i = 0; i < n; i++) {
intt v;
cin >> v;
cur = x + (cur - v);
cur = max(cur, x);
}
cout << cur << endl;
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
intt t = 1, buu = 1;
// cin >> t;
while(t--){
// cout << "Case #" << buu++ << ": ";
_();
}
}
