差分，注意ans开long long即可

#include <bits/stdc++.h>
using namespace std;
int diff[100005];
int a[100005];
typedef long long ll;

int main() {

    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    
    int n;
    cin>>n;
    for(int i=1;i<=n;++i)
    {
        cin>>a[i];
        a[i]-=1;
        diff[i]=a[i]-a[i-1];
    }
    //int ans=0;
    ll ans=0;
    for(int i=1;i<=n;++i)
    {
        if(diff[i]>0)ans+=diff[i];
    }
    cout<<ans;




    return 0;

}