#include <cstdio>

using namespace std;

int n, z;
int niz[101];

int main( void ){
    scanf( "%d", &n );
    
    for( int i = 0; i < n; ++i ) scanf( "%d", &niz[i] );
         
    int x = niz[n-1];
    int poc = x - n + 1;
    if( niz[0] > poc ) { z += niz[0]-poc; niz[0] = poc; }
    
    for( int i = n-2; i > 0; --i )
         if( niz[i] >= niz[i+1] ) { z += niz[i]-niz[i+1]+1; niz[i] = niz[i+1]-1; }

    printf( "%d\n", z );
    
    return 0;
}