dsa-competitive-programming/cses/1094.cpp

//URL - https://cses.fi/problemset/task/1094 
//CSES
#include <bits/stdc++.h>
using namespace std;

#ifdef LOCAL
#define dbg(x) cerr << #x << " = " << x << endl
#else
#define dbg(x)
#endif

#define ll long long
#define ld long double
#define ar array
#define all(x) (x).begin(), (x).end()
#define sza(x) (int)(x).size()

const ll INF = 1e18;
const int MOD = 1e9 + 7;

void solve() {
        ll n;
        cin>>n;
        vector<ll>s(n);
        ll count = 0;
        for(ll i =0;i<n;i++){
            cin>>s[i];
        }
        for(ll i=1;i<n;i++){
            ll diff = s[i-1] - s[i];
            if(diff>0) {
                count+=diff;
                s[i] = s[i-1];
        }
        }
        cout<<count;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(NULL);

    int tc = 1;
    // cin >> tc;
    while(tc--) solve();
}